Existing scholarship on East Asian international relations during the 7th-9th centuries primarily focuses on the conflicts between Silla, Baekje, and Goguryeo, and the Tang Dynasty's influence. However, the paper points to a gap in understanding Japan's perspective and its foreign relations during this period, particularly in the first half of the 7th century. While studies exist on Japanese fortifications, few connect the construction of these defenses to the changing international landscape of the time. This paper addresses this gap by exploring the construction of Japanese defensive fortifications, particularly Itojo and Onojo, examining their historical context, motivations, timing, and scale. The central argument is that Japan's sense of external pressure predated the Battle of Baekgang, and the construction of these fortifications reflects a proactive shift in Japan's foreign policy aimed at achieving a power balance with the Tang Dynasty and addressing both external and internal pressures.

The paper reviews existing literature on Tang-Japanese relations, focusing on the period before and after the Battle of Baekgang. It notes that while some scholars have examined the broader context of East Asian relations during this period, including Japan's interactions with the Tang Dynasty and its involvement in the Korean Peninsula conflicts, the specific connection between the construction of fortifications and Japan's perception of its international relationships remains unexplored. Similarly, while studies exist on the Japanese fortifications themselves, particularly concerning their construction techniques and locations, they mostly fail to analyze their strategic significance in the broader geopolitical landscape. The authors highlight that the existing literature often overlooks the earlier period of the 7th century, focusing primarily on the period after Baekgang, thereby missing a crucial element in Japan’s foreign policy evolution. The review also touches upon existing analyses of cultural and diplomatic exchanges between Japan and China, and how these exchanges intersected with the complex power dynamics on the Korean Peninsula.

The authors employ a historical research methodology, combining analysis of historical records (primarily the Nihongi) with archaeological evidence. The paper meticulously examines textual sources to establish timelines for the construction of Onojo and Itojo. They analyze the descriptions of the fortifications and their locations to infer their strategic purposes. By cross-referencing multiple sources and comparing different accounts, they attempt to reconstruct a detailed picture of the geopolitical environment in which these fortifications were built. The use of archaeological evidence from excavated sites provides additional data to corroborate the historical records and establish more precise timelines. The paper's methodology involves a detailed contextualization of the historical events, considering internal Japanese politics and the broader East Asian power struggles. The analysis draws upon various scholarly works on the Tang Dynasty, the Three Kingdoms of Korea, and Japanese history to understand the broader geopolitical landscape and Japan's place within it. The authors carefully evaluate different interpretations of the historical events and offer their own reasoned conclusions based on their analysis of the available evidence.

The paper's key findings challenge the traditional understanding of the timing of Japanese fortification construction. By analyzing the Nihongi and archaeological evidence, the authors suggest that Onojo's construction might have begun around AD 650, significantly predating the Battle of Baekgang (AD 663). This challenges the commonly held belief that the Battle of Baekgang was the primary catalyst for Japan's increased fortification efforts. The analysis of the Nihongi reveals a series of events and diplomatic exchanges that suggest growing tensions between Japan and Silla after Silla's increasing alignment with the Tang Dynasty and its adoption of Tang customs in AD 651. This changing diplomatic landscape indicates that Japan perceived growing external threats well before the military defeat at Baekgang. The detailed analysis of the construction of Onojo and Itojo reveals differences in construction style, with Onojo being of Joseon style, built by Baekje artisans, and Itojo of Chinese style, built under the supervision of the envoy Kibi no Makibi after his return from Tang. This difference highlights the cultural exchange and technological transfer between Japan and Tang, and its influence on Japan’s military infrastructure. The construction of Muzuki, Onojo, and Kiijo, creating a defensive ring around Dazaifu, further emphasizes Japan's concern for security. The later construction of Itojo, linked to the An-Shi Rebellion, shows Japan's continued vigilance against external threats. The paper further analyzes diplomatic exchanges between Japan and the Tang Dynasty, highlighting the limited success of these attempts at diplomatic resolution, and underscoring the importance of defensive preparations. The analysis reveals that Japan prioritized building fortifications over restoring diplomatic relations with the Tang Dynasty after Baekgang.

The findings demonstrate that Japan's construction of defensive fortifications was a complex process driven by a combination of external and internal factors. The shifting alliances and power struggles in East Asia, especially the growing alliance between Silla and the Tang Dynasty, were crucial external factors prompting Japan's proactive defensive measures. The construction of fortifications also served internal political objectives, strengthening centralized authority and suppressing regional power. The study's findings challenge the simplistic narrative of the Battle of Baekgang as the sole instigator of Japan's defensive building programs. By highlighting the early construction of Onojo, the research reveals a more nuanced understanding of Japan's proactive foreign policy during the Tang Dynasty. The changing architectural styles of Onojo and Itojo demonstrate the evolution of Japanese military engineering through interaction and exchange with other East Asian powers. The study also reveals the challenges faced by Japan in balancing its desire for diplomatic engagement with the need to prepare for potential military conflicts. The study offers a new perspective on Japan's role in East Asian relations during the Tang Dynasty and provides insights into the decision-making processes of the Japanese state during this formative period.

This paper significantly contributes to our understanding of Japan's early-7th-century foreign policy and the relationship between domestic development and international relations. The findings challenge existing assumptions by showing that Japan's fortification efforts began earlier than previously thought. The analysis highlights the complex interplay of external threats and internal political factors in shaping Japan's foreign policy. Future research could explore the social and economic impacts of these fortification projects on local communities, while also focusing on the comparison between Japanese fortifications and their counterparts in Korea and China during the same period.

The study relies heavily on textual evidence from the Nihongi, which, although an important primary source, may be subject to biases and inaccuracies. The archaeological evidence, while supportive, is not exhaustive, and future excavations could yield additional insights. Furthermore, the study primarily focuses on military aspects of Japan’s response to external pressures, leaving room for further research exploring other facets of Japan’s foreign policy and diplomatic strategies during this period.