Yamato Intervention and the Division of Unakami no Kuni:A Structural Model of Eastern State Formationーー上海上国造と下海上国造

Introduction

The purpose of this study is to reconstruct the historical formation, political reorganization, and territorial dynamics of the Unakami no Kuni lineage—later known as the Shanghai-Unakami and ShimoUnakami kuni no miyatsuko—in the central region of ancient Kazusa Province (present-day Chiba Prefecture). While previous scholarship has treated these lineages primarily as local hereditary officials, this paper argues that they originated from a large-scale coastal polity that controlled both maritime routes in Tokyo Bay and inland water transportation along the Yōrō and Obitsu river systems. By the fifth century, this polity had grown sufficiently powerful to require political division, resulting in the bifurcation of the Unakami lineage into upper and lower branches under the influence of the Yamato court.

Archaeological evidence provides the first key to understanding this transformation. The massive tumuli constructed along the terrace edges of the Yōrō River basin—particularly the Anegasaki Kofun cluster—indicate the presence of a dominant coastal chieftaincy with extensive control over regional traffic and ritual authority. The discovery of the Inaridai No.1 iron sword, inscribed with references to the Yamato polity, further demonstrates early and direct contact between the Unakami leadership and the central court during the late third to fourth centuries.

From the late fourth to early fifth century, the Unakami polity reached its zenith, functioning as a unified regional power with influence extending across the Kantō region. However, during the latter half of the fifth century—likely in the political aftermath of the Kōsaka no Ō and Oshikuma no Ō incident—the Yamato court undertook a deliberate reorganization of eastern regional authorities. As a result, the Unakami polity was divided into two hereditary offices: the Shanghai-Unakami kuni no miyatsuko (the senior branch) and the ShimoUnakami kuni no miyatsuko (the junior branch). Genealogical traditions consistently trace both lineages to Takehiratori no Mikoto, a descendant of Ame-no-hohi no Mikoto, thereby preserving the memory of an originally unified coastal polity.

The sixth century witnessed further administrative restructuring. The emergence of the Musha kuni no miyatsuko—strategically inserted between the territories of the Unakami lineages—suggests direct intervention by the Yamato court to secure military and maritime control over the region. Simultaneously, the southern Kikuma kuni no miyatsuko rose as an autonomous local power, reflecting increasing fragmentation of the original Unakami sphere.

By the late sixth to seventh century, the authority of the Unakami lineages had diminished, and the establishment of Kazusa Province under the ritsuryō system formalized the final stage of regional administrative integration. The long-term trajectory of the Unakami polity—from unified coastal chieftaincy to divided hereditary offices, and finally to provincial incorporation—reveals a dynamic interplay between local power structures and central political strategies.

This paper therefore advances three central arguments. First, the Unakami polity should be understood not as a minor local lineage but as a large-scale coastal federation whose division reflects deliberate political intervention by the Yamato court. Second, genealogical and archaeological evidence together demonstrate that the Shanghai-Unakami and ShimoUnakami lineages originated from a single ancestral authority whose fragmentation was tied to fifth-century court politics. Third, the insertion of the Musha kuni no miyatsuko and the rise of the Kikuma lineage illustrate the Yamato court’s multi-layered strategy to secure maritime transportation, military oversight, and regional governance in the Bōsō Peninsula.

By integrating archaeological data, genealogical traditions, and political history, this study proposes a new structural model for understanding the formation and transformation of coastal hereditary offices in eastern Japan.

 

Chapter 1

The Formation of Unakami no Kuni and Its Geopolitical Environment**

1.1 Coastal Geography and the Strategic Position of Central Kazusa

The emergence of Unakami no Kuni must be understood within the unique coastal and fluvial geography of the central Bōsō Peninsula. The region is defined by two major river systems—the Yōrō River and the Obitsu River—both flowing into the innermost recess of Tokyo Bay. These waterways created a dual transportation network: maritime routes connecting the peninsula to the Kinai region, and inland watercourses enabling movement of goods and people deep into the hinterland.

The terrace edges along the Yōrō River, where the Anegasaki Kofun cluster is located, provided ideal conditions for constructing monumental tumuli. Their placement allowed direct visual and logistical control over both maritime and riverine traffic. This combination of coastal access and inland reach made the region one of the most strategically valuable zones in eastern Japan during the late Yayoi and early Kofun periods.

In this environment, a powerful coastal chieftaincy emerged—one capable of coordinating maritime transport, ritual authority, and regional governance. This polity, later remembered as Unakami no Kuni, formed the nucleus from which the Shanghai-Unakami and ShimoUnakami hereditary offices would eventually develop.

1.2 Archaeological Foundations of a Coastal Polity

Archaeological evidence provides the earliest and most concrete indicators of the scale and political character of the Unakami polity. The Anegasaki Kofun cluster, consisting of large keyhole-shaped tumuli constructed along the Yōrō River terraces, demonstrates the presence of a ruling lineage with substantial labor mobilization capacity and ritual authority.

The chronological placement of these tumuli—late third to early fourth century—coincides with the period in which regional chieftains across eastern Japan began to establish formalized relations with the Yamato court. The discovery of the Inaridai No.1 iron sword, inscribed with references to the Yamato polity, is particularly significant. It indicates not only early contact but also political alignment between the Unakami leadership and the central court.

This archaeological corpus suggests that Unakami no Kuni was not a minor local group but a large-scale coastal federation capable of integrating maritime and inland resources. Its leaders were positioned to act as intermediaries between eastern regional powers and the Yamato state.

1.3 Maritime Networks and the Rise of a Bay-Control Polity

The geopolitical strength of Unakami no Kuni derived from its ability to control the maritime corridor of Tokyo Bay. During the third to fifth centuries, the bay served as the primary gateway linking the Kantō region to the Kinai political center. Ships traveling from the eastern provinces necessarily passed through the inner bay, making the Unakami territory a critical node in long-distance communication and transport.

The Unakami polity’s control over this corridor would have enabled:

• regulation of maritime traffic
• taxation or redistribution of goods transported along the bay
• coordination of ritual exchanges with the Yamato court
• strategic oversight of military movements

Inland water transport along the Yōrō and Obitsu rivers further amplified this advantage. Goods arriving by sea could be moved inland without reliance on overland routes, allowing the Unakami leadership to integrate coastal and interior communities under a single political framework.

This dual maritime–fluvial network is a defining feature of Unakami no Kuni and explains its rapid rise as a regional power.

1.4 Political Consolidation and the Emergence of a Unified Coastal Polity

By the late fourth century, the Unakami polity appears to have consolidated multiple local lineages into a single chieftaincy. The scale of the Anegasaki tumuli suggests a level of political integration comparable to other major regional polities such as the Kibi, Izumo, and Tsukushi confederations.

The genealogical tradition tracing the Unakami lineage to Ame-no-hohi no Mikoto—a deity associated with ritual authority and political mediation—further supports the interpretation of Unakami no Kuni as a unified coastal federation. The ancestral figure Takehiratori no Mikoto is consistently presented as the progenitor of the later Shanghai-Unakami and ShimoUnakami lineages, indicating that both branches originated from a single political authority.

This consolidation likely occurred in response to increasing contact with the Yamato court. As the central state expanded its influence eastward, regional polities such as Unakami no Kuni strengthened their internal structures to negotiate with the court from a position of stability.

1.5 Early Relations with the Yamato Court

The relationship between Unakami no Kuni and the Yamato court was established earlier than most eastern polities. The Inaridai sword inscription demonstrates that by the late third or early fourth century, Unakami leaders were already participating in the exchange networks that linked regional chieftains to the central state.

Such early contact suggests that the Yamato court recognized the strategic importance of the Unakami polity. Control over Tokyo Bay was essential for maintaining communication with eastern provinces, and the Unakami leadership’s cooperation would have been indispensable for the court’s political and military ambitions.

This early alignment laid the foundation for later political interventions by the Yamato court, including the eventual division of the Unakami lineage into upper and lower hereditary offices.

1.6 Summary: The Geopolitical Logic of Unakami no Kuni

The formation of Unakami no Kuni can be summarized as the product of four interlocking factors:

1. Coastal geography – Direct access to Tokyo Bay and inland river systems.
2. Archaeological evidence of political centralization – Large-scale tumuli indicating a unified chieftaincy.
3. Maritime–fluvial transportation networks – Control over regional traffic and resource flows.
4. Early integration into Yamato political structures – Participation in court-centered exchange and ritual systems.

These factors combined to produce a powerful coastal polity whose later division into Shanghai-Unakami and ShimoUnakami hereditary offices reflects both internal growth and external political intervention.

Chapter 2

The Division of Unakami no Kuni and Political Intervention by the Yamato Court**

2.1 The Political Context of Fifth-Century Eastern Japan

The division of Unakami no Kuni into the Shanghai-Unakami and ShimoUnakami hereditary offices occurred within a broader transformation of regional governance in eastern Japan during the late fifth century. This period was marked by intensified political centralization under the Yamato court, accompanied by a series of military and ritual interventions aimed at consolidating authority over distant provinces.

The Kōsaka no Ō and Oshikuma no Ō incident—an internal conflict within the royal family—triggered a reorganization of regional chieftaincies across eastern Japan. Although the event itself was centered in the Kinai region, its political consequences extended outward, prompting the court to reassess and restructure the authority of powerful coastal polities such as Unakami no Kuni.

The Yamato court’s strategy in the east combined three elements:

1. Political punishment and redistribution of authority
2. Securing maritime transportation routes
3. Fragmenting large regional polities to prevent autonomous expansion

The division of Unakami no Kuni must be understood as part of this multi-layered strategy.

2.2 Internal Growth and the Pressures of Political Expansion

By the early fifth century, Unakami no Kuni had grown into a large-scale coastal federation with influence extending across the Kantō region. Its leaders controlled maritime corridors, inland water transport, and ritual authority. This internal expansion created pressures that made political division increasingly likely.

Two internal factors contributed to the eventual bifurcation:

(1) Enlargement of the chieftaincy and territorial complexity

The consolidation of multiple local lineages under a single Unakami authority increased administrative demands. As the polity expanded, internal differentiation became necessary to maintain governance over a widening territory.

(2) Succession dynamics within the ruling lineage

Genealogical traditions suggest that the Unakami lineage maintained a hierarchical structure in which senior and junior branches coexisted. The emergence of the Shanghai-Unakami (upper) and ShimoUnakami (lower) branches reflects this internal differentiation.

These internal pressures alone, however, do not fully explain the timing or nature of the division. External political intervention by the Yamato court played a decisive role.

2.3 Yamato Court Intervention and the Political Logic of Division

The Yamato court’s involvement in the division of Unakami no Kuni can be reconstructed through three interrelated mechanisms.

(1) Post-conflict political reorganization

Following the Kōsaka no Ō and Oshikuma no Ō incident, the court sought to stabilize regional governance by redistributing authority among loyal lineages. Large coastal polities were particularly targeted due to their strategic importance.

(2) Securing control over Tokyo Bay

Unakami no Kuni’s dominance over the inner bay posed both an opportunity and a risk for the Yamato court. While cooperation with Unakami leaders facilitated communication with eastern provinces, excessive autonomy could threaten central authority. Dividing the polity into upper and lower branches reduced the risk of unified regional resistance.

(3) Fragmentation as a tool of governance

The division of Unakami no Kuni fits a broader pattern observed in other regions, where the court intentionally fragmented large chieftaincies to prevent the emergence of independent regional powers. The creation of the Shanghai-Unakami and ShimoUnakami hereditary offices reflects this administrative logic.

Thus, the division was not merely a genealogical event but a deliberate political act embedded within the Yamato court’s strategy for eastern governance.

2.4 Genealogical Evidence for a Politically Engineered Division

The genealogical traditions preserved in later sources consistently trace both the Shanghai-Unakami and ShimoUnakami lineages to Takehiratori no Mikoto, a descendant of Ame-no-hohi no Mikoto. This shared ancestry indicates that the two offices originated from a single political authority.

The genealogical sequence:

• Ame-no-hohi no Mikoto
• Takehiratori no Mikoto
• Oshitatakatarahi no Mikoto (Shanghai-Unakami ancestor)
• Isosame no Sukune
• Kutsugi no Ason (ShimoUnakami ancestor)

reveals a clear hierarchical structure in which the Shanghai-Unakami lineage represents the senior branch and the ShimoUnakami lineage the junior branch. This hierarchical bifurcation is consistent with a politically engineered division rather than a natural genealogical divergence.

The genealogical model thus supports the interpretation that the Yamato court intervened to divide the Unakami polity into two hereditary offices.

2.5 The Insertion of Musha Kuni no Miyatsuko: A Deliberate “Wedge”

One of the most striking pieces of evidence for Yamato intervention is the establishment of the Musha kuni no miyatsuko in the early sixth century. The Musha territory lies geographically between the Shanghai-Unakami and ShimoUnakami domains, suggesting intentional placement by the central court.

The Musha office served three strategic purposes:

1. Fragmenting the Unakami sphere – Preventing reunification of the upper and lower branches.
2. Establishing direct court oversight – Acting as a centrally controlled administrative node.
3. Securing military and maritime routes – Ensuring safe passage along the Tokyo Bay corridor.

The insertion of Musha represents a classic example of the Yamato court’s use of “administrative wedges” to weaken large regional polities.

2.6 The Rise of Kikuma Kuni no Miyatsuko and Regional Fragmentation

While Musha represents direct intervention, the rise of the Kikuma kuni no miyatsuko in southern Kazusa reflects a different dynamic: the emergence of autonomous local powers. The Kikuma lineage appears to have consolidated its authority independently, taking advantage of the weakening Unakami sphere.

This development contributed to the fragmentation of the original Unakami polity and further reduced the influence of the Shanghai-Unakami and ShimoUnakami offices.

The combination of:

• Yamato intervention (Musha)
• Local autonomy (Kikuma)
• Internal differentiation (Shanghai-Unakami and ShimoUnakami)

produced a complex political landscape in which the original Unakami federation was permanently divided.

2.7 Summary: Division as a Multi-Causal Political Process

The division of Unakami no Kuni was not a simple genealogical split but a multi-causal political process shaped by:

1. Internal growth and administrative complexity
2. Succession dynamics within the Unakami lineage
3. Post-conflict reorganization by the Yamato court
4. Strategic control of maritime transportation
5. Deliberate fragmentation through the insertion of Musha
6. Regional autonomy through the rise of Kikuma

This chapter demonstrates that the division of Unakami no Kuni reflects the intersection of local political evolution and central state intervention. The resulting configuration—Shanghai-Unakami, ShimoUnakami, Musha, and Kikuma—formed the foundation for the later administrative structure of Kazusa Province under the ritsuryō system.

**Chapter 3

Genealogical Structure and the Political Hierarchy of the Unakami Lineages**

3.1 Genealogy as a Political Framework

The genealogical traditions surrounding the Unakami lineages are not merely records of familial descent; they constitute a political framework that reflects the hierarchical organization of coastal authority in ancient Kazusa. In early Japanese polities, genealogies served as instruments for legitimizing territorial control, ritual authority, and hereditary office. The Unakami genealogical corpus must therefore be interpreted as a political document that encodes the structure of a once-unified coastal federation.

The consistent attribution of both the Shanghai-Unakami and ShimoUnakami kuni no miyatsuko to Takehiratori no Mikoto, a descendant of Ame-no-hohi no Mikoto, indicates that the two offices originated from a single ancestral authority. This shared lineage is not incidental; it reflects the memory of a unified Unakami polity whose division was later formalized through political intervention by the Yamato court.

3.2 The Ancestral Line: From Ame-no-hohi to Takehiratori

The genealogical foundation of the Unakami lineages begins with Ame-no-hohi no Mikoto, a deity associated with mediation, ritual authority, and political negotiation. His descendants appear frequently in the genealogies of eastern hereditary offices, suggesting a widespread tradition of attributing regional authority to this lineage.

Within this broader mytho-political framework, Takehiratori no Mikoto emerges as the specific ancestral figure of the Unakami polity. His position in the genealogical sequence marks the point at which the lineage transitions from mythic ancestry to historically grounded political authority.

The genealogical model can be summarized as follows:

• Ame-no-hohi no Mikoto – Mythic ancestor associated with ritual authority
• Takehiratori no Mikoto – Foundational ancestor of the Unakami polity
• Oshitatakatarahi no Mikoto – Ancestor of the Shanghai-Unakami lineage
• Isosame no Sukune – Intermediate figure
• Kutsugi no Ason – Ancestor of the ShimoUnakami lineage

This structure reveals a clear hierarchical differentiation between the senior (Shanghai-Unakami) and junior (ShimoUnakami) branches.

3.3 Hierarchical Bifurcation: Senior and Junior Branches

The genealogical bifurcation between the Shanghai-Unakami and ShimoUnakami lineages reflects a political hierarchy rather than a simple familial divergence. The Shanghai-Unakami lineage, descending from Oshitatakatarahi no Mikoto, occupies the senior position, while the ShimoUnakami lineage, descending from Kutsugi no Ason, represents the junior branch.

This hierarchical structure is consistent with the political logic of the Unakami polity:

1. The senior branch (Shanghai-Unakami) – Controlled the northern portion of the Unakami sphere – Maintained earlier and stronger ties with the Yamato court – Preserved the largest tumuli and ritual centers
2. The junior branch (ShimoUnakami) – Occupied the southern portion of the Unakami sphere – Emerged as a derivative authority following political division – Functioned as a subordinate hereditary office within the broader Unakami framework

The genealogical hierarchy thus mirrors the territorial and political hierarchy of the Unakami polity.

3.4 Genealogy as Evidence of Political Division

The genealogical structure provides crucial evidence for understanding the political division of Unakami no Kuni. The presence of a single ancestral figure (Takehiratori no Mikoto) followed by a bifurcation into senior and junior branches suggests that the division was not the result of independent lineage development but of deliberate political restructuring.

This interpretation aligns with the historical context of the late fifth century, when the Yamato court undertook a systematic reorganization of regional chieftaincies. The division of the Unakami lineage into upper and lower hereditary offices reflects the court’s strategy of fragmenting large coastal polities to prevent autonomous expansion.

Thus, genealogy serves as a structural record of political intervention.

3.5 The Political Role of Genealogical Legitimacy

In early Japanese governance, genealogical legitimacy was essential for maintaining hereditary office. The Unakami lineages used their descent from Ame-no-hohi no Mikoto and Takehiratori no Mikoto to assert authority over maritime and inland territories. This legitimacy was reinforced through:

• construction of monumental tumuli
• participation in ritual exchanges with the Yamato court
• control over transportation networks
• hereditary succession within a politically recognized lineage

The genealogical hierarchy between the Shanghai-Unakami and ShimoUnakami offices thus functioned as a political mechanism for organizing authority within the divided Unakami sphere.

3.6 The Musha and Kikuma Lineages in the Genealogical Landscape

The insertion of the Musha kuni no miyatsuko and the rise of the Kikuma lineage further complicate the genealogical landscape. Unlike the Shanghai-Unakami and ShimoUnakami lineages, Musha and Kikuma do not share descent from Takehiratori no Mikoto. Their genealogical independence reflects their distinct political roles:

1. Musha lineage – Introduced by the Yamato court as a centrally controlled administrative wedge – Genealogically separate from the Unakami sphere – Functioned to fragment and supervise the Unakami territories
2. Kikuma lineage – Emerged as an autonomous local power in southern Kazusa – Genealogically independent – Represented regional fragmentation rather than central intervention

The genealogical independence of Musha and Kikuma underscores the political fragmentation of the original Unakami federation.

3.7 Genealogy as a Mirror of Political Hierarchy

The genealogical structure of the Unakami lineages reveals a multi-layered political hierarchy:

1. Mythic authority – Descent from Ame-no-hohi no Mikoto
2. Foundational authority – Takehiratori no Mikoto as the progenitor of the unified Unakami polity
3. Senior authority – Shanghai-Unakami lineage
4. Junior authority – ShimoUnakami lineage
5. Central oversight – Musha lineage
6. Regional autonomy – Kikuma lineage

This hierarchy reflects the interplay between local political evolution and central state intervention.

3.8 Summary: Genealogy as Political Structure

The genealogical traditions of the Unakami lineages encode the political structure of a once-unified coastal federation. The bifurcation into senior and junior branches reflects deliberate political intervention by the Yamato court, while the insertion of Musha and the rise of Kikuma illustrate the fragmentation of regional authority.

Genealogy thus serves not only as a record of descent but as a structural map of political hierarchy, territorial control, and administrative reorganization in ancient Kazusa.

Chapter 4

Geopolitical Reorganization and the Integration of the Kuni no Miyatsuko System**

4.1 From Coastal Federation to Fragmented Territorial Order

The division of Unakami no Kuni into the Shanghai-Unakami and ShimoUnakami hereditary offices initiated a profound territorial transformation across central Kazusa. What had once been a unified coastal federation—anchored by control of Tokyo Bay and the Yōrō–Obitsu river systems—became a fragmented political landscape composed of multiple hereditary authorities with differing origins, functions, and degrees of autonomy.

This fragmentation was not merely the result of internal lineage dynamics; it reflected a broader geopolitical reorganization driven by the Yamato court’s strategic interests. The insertion of the Musha kuni no miyatsuko and the rise of the Kikuma lineage further altered the territorial balance, producing a multi-centered configuration that replaced the earlier Unakami hegemony.

The resulting territorial order can be characterized as:

1. Bifurcated Unakami sphere – Shanghai-Unakami (senior) – ShimoUnakami (junior)
2. Central administrative wedge – Musha kuni no miyatsuko
3. Autonomous southern polity – Kikuma kuni no miyatsuko

This configuration formed the geopolitical foundation upon which the later provincial system would be constructed.

4.2 Strategic Logic of Yamato Territorial Intervention

The Yamato court’s territorial interventions in Kazusa were guided by three strategic imperatives:

(1) Securing maritime corridors in Tokyo Bay

Control of the inner bay was essential for maintaining communication with eastern provinces. Fragmenting the Unakami sphere prevented any single lineage from monopolizing maritime transport.

(2) Establishing direct administrative oversight

The Musha office served as a centrally controlled node, enabling the court to supervise coastal and inland routes without relying solely on local hereditary authorities.

(3) Preventing regional consolidation

By dividing Unakami and supporting the rise of Kikuma, the court ensured that no unified coastal polity could challenge central authority.

These interventions transformed Kazusa from a region dominated by a single coastal federation into a strategically partitioned territory aligned with Yamato administrative priorities.

4.3 Territorial Domains of the Four Hereditary Offices

The geopolitical reorganization of Kazusa can be mapped through the territorial domains of the four hereditary offices.

(1) Shanghai-Unakami (Upper Unakami)

• Controlled the northern coastal zone
• Maintained earlier ties with the Yamato court
• Preserved the largest tumuli and ritual centers
• Functioned as the senior authority within the divided Unakami sphere

(2) ShimoUnakami (Lower Unakami)

• Occupied the southern portion of the original Unakami territory
• Emerged as a junior branch following political division
• Held reduced territorial and ritual authority compared to Shanghai-Unakami

(3) Musha

• Strategically inserted between the two Unakami domains
• Genealogically independent from the Unakami lineage
• Served as a central administrative wedge
• Ensured direct oversight of maritime and inland routes

(4) Kikuma

• Located in southern Kazusa
• Emerged autonomously rather than through central intervention
• Represented regional fragmentation and local consolidation
• Contributed to the permanent dissolution of the unified Unakami polity

This territorial configuration reflects the interplay between local political evolution and central administrative strategy.

4.4 Geopolitical Consequences of Fragmentation

The fragmentation of Unakami no Kuni produced several long-term geopolitical consequences:

(1) Loss of unified coastal authority

The Unakami lineage no longer controlled the entire Tokyo Bay corridor. This reduced its ability to act as a regional intermediary and weakened its political leverage.

(2) Emergence of multi-centered governance

Kazusa became a region governed by multiple hereditary offices, each with distinct origins and political functions. This multi-centered structure facilitated Yamato oversight.

(3) Increased administrative complexity

The coexistence of four hereditary offices required new mechanisms for coordinating ritual obligations, transportation logistics, and military mobilization.

(4) Foundations for provincial integration

The fragmented territorial order created conditions conducive to the later establishment of Kazusa Province under the ritsuryō system.

Thus, fragmentation was not merely a political event but a structural transformation that reshaped the region’s administrative landscape.

4.5 Integration into the Ritsuryō Provincial System

The final stage of geopolitical reorganization occurred with the establishment of Kazusa Province during the late sixth to seventh centuries. The ritsuryō system replaced hereditary offices with centrally appointed provincial officials, integrating regional governance into a standardized administrative framework.

The transition from kuni no miyatsuko to provincial administration involved:

(1) Territorial consolidation

The fragmented domains of Shanghai-Unakami, ShimoUnakami, Musha, and Kikuma were reorganized into districts (gun) under provincial authority.

(2) Administrative standardization

Provincial governors (kokushi) replaced hereditary chieftains, implementing uniform legal and fiscal practices.

(3) Ritual centralization

Local ritual authority—once held by Unakami lineages—was incorporated into the provincial shrine system.

(4) Transportation and military integration

Maritime and inland routes formerly controlled by Unakami were placed under provincial oversight, ensuring direct alignment with central policy.

The ritsuryō system thus completed the transformation initiated by the division of Unakami no Kuni.

4.6 Structural Model of Territorial Transformation

The territorial transformation of Kazusa can be represented as a four-stage structural model:

1. Unified coastal federation (3rd–4th century) – Unakami no Kuni controls maritime and inland networks.
2. Peak expansion and early Yamato alignment (early 5th century) – Unakami becomes a major regional power.
3. Political division and fragmentation (late 5th–6th century) – Shanghai-Unakami, ShimoUnakami, Musha, and Kikuma emerge.
4. Provincial integration (late 6th–7th century) – Kazusa Province established under ritsuryō administration.

This model illustrates how local political evolution and central intervention jointly produced the administrative geography of early Japan.

4.7 Summary: Geopolitical Reorganization as a Path to Provincial Integration

The division of Unakami no Kuni initiated a process of geopolitical reorganization that reshaped the territorial and administrative landscape of central Kazusa. Through strategic intervention, the Yamato court fragmented the coastal federation, inserted new administrative nodes, and facilitated the rise of autonomous local powers. This multi-centered territorial order provided the structural foundation for the later integration of Kazusa into the ritsuryō provincial system.

The transformation of Unakami no Kuni thus exemplifies the broader pattern of state formation in early Japan, in which local polities were gradually incorporated into a centralized administrative framework through a combination of political intervention, territorial restructuring, and genealogical redefinition.

Chapter 5

The Historical Significance of the Unakami Lineages and a Structural Model of Eastern Governance**

5.1 Reassessing the Role of Coastal Polities in Early Japanese State Formation

The Unakami lineages—Shanghai-Unakami and ShimoUnakami—represent one of the most important examples of how coastal chieftaincies shaped the political landscape of eastern Japan during the Kofun and Asuka periods. Their historical significance lies not only in their territorial control over Tokyo Bay and the Yōrō–Obitsu river systems but also in their role as intermediaries between regional powers and the Yamato court.

Traditional scholarship has tended to treat eastern hereditary offices as peripheral actors in the formation of the early Japanese state. However, the Unakami case demonstrates that coastal polities were central to the Yamato court’s strategy for integrating distant regions. Their early alignment with the court, their subsequent division, and their eventual incorporation into the ritsuryō provincial system reveal a dynamic process of negotiation, intervention, and structural transformation.

The Unakami lineages thus provide a crucial lens through which to understand the mechanisms of eastern governance.

5.2 Coastal Authority as a Strategic Asset in Eastern Japan

The strategic value of the Unakami polity derived from its control over maritime and inland transportation networks. During the third to sixth centuries, Tokyo Bay served as the primary gateway linking the Kantō region to the Kinai political center. The Unakami chieftains’ ability to regulate this corridor gave them significant leverage in regional politics.

Three forms of authority can be identified:

(1) Maritime authority

Control over ship routes, coastal landing points, and bay traffic.

(2) Fluvial authority

Management of inland water transport along the Yōrō and Obitsu rivers.

(3) Ritual authority

Maintenance of large tumuli and participation in court-centered ritual exchanges.

These forms of authority positioned the Unakami lineage as a regional power whose cooperation was essential for the Yamato court’s eastern expansion.

5.3 The Unakami Lineages as Intermediaries in Yamato–Eastern Relations

The Unakami lineages functioned as intermediaries between the Yamato court and eastern regional powers. Their early contact with the court—attested by the Inaridai sword inscription—suggests that they played a role in facilitating communication, ritual exchange, and military coordination.

This intermediary function can be understood through three mechanisms:

(1) Transmission of court authority

Unakami leaders acted as conduits through which Yamato political influence reached eastern communities.

(2) Regulation of regional traffic

Their control over maritime and inland routes allowed them to manage the flow of goods, people, and information.

(3) Integration of local polities

By consolidating smaller lineages under their authority, the Unakami chieftains created a regional structure compatible with Yamato administrative goals.

Thus, the Unakami lineages were not passive recipients of court authority but active participants in shaping the political geography of eastern Japan.

5.4 Division and Fragmentation as Instruments of State Formation

The division of Unakami no Kuni into senior and junior branches, followed by the insertion of Musha and the rise of Kikuma, illustrates how fragmentation served as a tool of state formation. The Yamato court used division to weaken large regional polities, prevent autonomous expansion, and establish direct administrative oversight.

This process reflects a broader pattern in early Japanese governance:

1. Identify powerful regional polities
2. Fragment them through genealogical or territorial division
3. Insert centrally controlled administrative nodes
4. Integrate fragmented territories into the provincial system

The Unakami case provides a clear example of this pattern, demonstrating how the court transformed a unified coastal federation into a multi-centered administrative landscape.

5.5 Toward a Structural Model of Eastern Governance

Based on the Unakami case, a structural model of eastern governance can be proposed. This model identifies the mechanisms through which the Yamato court integrated eastern regions into the emerging state.

(1) Coastal gateway control

Dominant coastal polities (e.g., Unakami) served as initial points of contact.

(2) Political alignment and ritual exchange

Early alliances were established through ritual and symbolic interactions.

(3) Strategic fragmentation

Large polities were divided to prevent regional autonomy.

(4) Administrative insertion

New hereditary offices (e.g., Musha) were introduced to secure oversight.

(5) Regional consolidation

Autonomous local powers (e.g., Kikuma) emerged within the fragmented landscape.

(6) Provincial integration

The ritsuryō system reorganized fragmented territories into standardized provinces.

This model provides a framework for understanding how the Yamato court transformed eastern Japan from a mosaic of regional polities into a unified administrative system.

5.6 The Unakami Lineages in the Broader Context of Eastern Polities

The Unakami case must be situated within the broader context of eastern hereditary offices, including:

• Katori
• Kashima
• Izumi
• Hitachi
• Musashi
• Shimotsuke
• Kōzuke

Many of these lineages exhibit similar patterns of early alignment, political fragmentation, and eventual provincial integration. The Unakami lineages thus represent a microcosm of eastern state formation, providing a detailed case study through which larger structural patterns can be analyzed.

5.7 Historical Significance of the Unakami Lineages

The historical significance of the Unakami lineages can be summarized in three points:

(1) They exemplify the role of coastal polities in early state formation.

Their control over maritime and inland networks made them indispensable to the Yamato court.

(2) Their division reveals the mechanisms of political intervention.

The bifurcation into senior and junior branches reflects deliberate court strategy.

(3) Their transformation illustrates the process of provincial integration.

The Unakami sphere’s fragmentation created the conditions necessary for the establishment of Kazusa Province.

Thus, the Unakami lineages occupy a central place in the history of eastern governance.

5.8 Conclusion: The Unakami Lineages as a Structural Key to Eastern State Formation

The Unakami lineages provide a structural key to understanding the formation of the early Japanese state in eastern Japan. Their rise, division, fragmentation, and integration reflect the interplay between local political evolution and central administrative strategy.

By analyzing the Unakami case, this study proposes a new structural model of eastern governance—one that emphasizes the importance of coastal authority, genealogical hierarchy, territorial fragmentation, and administrative integration.

The Unakami lineages thus stand not only as historical actors but as structural indicators of how the Yamato court transformed regional polities into components of a unified state.

Chapter 6

Conclusion**

6.1 Reconstructing the Historical Trajectory of Unakami no Kuni

This study has reconstructed the formation, division, and transformation of Unakami no Kuni—a powerful coastal polity that dominated the inner Tokyo Bay region from the late third to sixth centuries. Through archaeological evidence, genealogical traditions, and political analysis, we have demonstrated that the Unakami lineages were not peripheral actors but central components in the Yamato court’s strategy for governing eastern Japan.

The historical trajectory of Unakami no Kuni can be summarized as follows:

1. Formation of a unified coastal federation – Anchored by control of maritime and fluvial networks – Supported by monumental tumuli and early Yamato contact
2. Peak expansion and regional influence – Functioning as a large-scale chieftaincy with wide territorial reach
3. Political division under Yamato intervention – Bifurcation into Shanghai-Unakami (senior) and ShimoUnakami (junior) – Fragmentation reinforced by the insertion of Musha and rise of Kikuma
4. Geopolitical reorganization – Transformation into a multi-centered territorial landscape
5. Integration into the ritsuryō provincial system – Establishment of Kazusa Province and dissolution of hereditary offices

This trajectory illustrates the dynamic interplay between local political evolution and central administrative strategy.

6.2 The Structural Role of Coastal Polities in Eastern State Formation

The Unakami case reveals that coastal polities played a structural role in the formation of the early Japanese state. Their control over maritime corridors, inland water transport, and ritual centers made them indispensable to the Yamato court’s eastern expansion.

Three structural functions can be identified:

(1) Gateways of political integration

Coastal polities served as initial points of contact between the Yamato court and eastern regions.

(2) Mediators of regional governance

They transmitted court authority, regulated traffic, and integrated local communities.

(3) Targets of strategic fragmentation

Their power made them subjects of deliberate political division to prevent autonomous expansion.

The Unakami lineages exemplify these functions with exceptional clarity.

6.3 Genealogy as a Political Instrument

The genealogical structure of the Unakami lineages—rooted in Ame-no-hohi no Mikoto and Takehiratori no Mikoto—served as a political instrument for legitimizing authority. The bifurcation into senior and junior branches reflects not natural lineage divergence but deliberate political engineering.

Genealogy thus functioned as:

• a mechanism for organizing authority
• a record of political intervention
• a structural map of territorial hierarchy

The Unakami genealogical corpus provides rare insight into how hereditary offices were reshaped to align with central administrative goals.

6.4 Fragmentation as a Mechanism of State Formation

The division of Unakami no Kuni demonstrates that fragmentation was a key mechanism in early Japanese state formation. The Yamato court used division to weaken large regional polities, insert centrally controlled administrative nodes, and facilitate provincial integration.

This mechanism operated through:

1. Genealogical division
2. Territorial partitioning
3. Administrative insertion (Musha)
4. Encouragement of local autonomy (Kikuma)
5. Provincial consolidation

The Unakami case thus provides a structural model for understanding how the Yamato court transformed regional polities into components of a unified state.

6.5 A Structural Model of Eastern Governance

Based on the Unakami case, this study proposes a structural model of eastern governance:

1. Coastal gateway control
2. Early political alignment
3. Strategic fragmentation of large polities
4. Insertion of central administrative nodes
5. Emergence of autonomous local powers
6. Provincial integration under the ritsuryō system

This model offers a framework for analyzing other eastern hereditary offices and contributes to a broader understanding of early Japanese state formation.

6.6 Historical Significance of the Unakami Lineages

The Unakami lineages hold historical significance for three reasons:

(1) They illuminate the role of coastal authority in early state formation.

Their control over Tokyo Bay made them central to Yamato political strategy.

(2) They reveal the mechanisms of political intervention and fragmentation.

Their division reflects deliberate administrative restructuring.

(3) They provide a structural key to understanding provincial integration.

Their transformation laid the foundation for the establishment of Kazusa Province.

Thus, the Unakami lineages serve as a critical case study for reconstructing the political geography of early Japan.

6.7 Future Directions for Research

This study opens several avenues for further research:

1. Comparative analysis with other coastal polities – Katori, Kashima, Musashi, Hitachi, and Shimotsuke
2. Integration of paleoenvironmental data – Reconstruction of ancient coastlines and river systems
3. Quantitative modeling of transportation networks – Maritime and fluvial traffic simulations
4. Reevaluation of tumuli distribution and hierarchy – Spatial analysis of political centers
5. Examination of ritual exchange networks – Connections between Unakami and Yamato ritual systems

These directions will deepen our understanding of eastern governance and the formation of the early Japanese state.

6.8 Final Statement

The Unakami lineages—Shanghai-Unakami, ShimoUnakami, Musha, and Kikuma—represent a microcosm of the processes through which the Yamato court transformed regional polities into components of a centralized state. Their rise, division, fragmentation, and integration reveal the structural mechanisms of early Japanese governance.

By analyzing the Unakami case, this study provides a new theoretical model for understanding eastern state formation—one that emphasizes coastal authority, genealogical hierarchy, territorial fragmentation, and administrative integration.

The Unakami lineages thus stand as a structural key to the political history of ancient Japan.

References

Primary Sources (Classical Texts and Ancient Documents)

• Kojiki. Various editions.
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• Engishiki. Vols. 1–10.
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