古事記と日本書紀が生まれた事情Kojiki and Nihon Shoki's Role in Forging Early Japan

Japan’s earliest historical texts, the Kojiki (712) and the Nihon Shoki (720), both systematically compile ancient myths and traditions, but why were two separate historical accounts needed in roughly the same period? Traditionally, the distinction has been explained by characterizing the Kojiki as inward-facing—intended primarily for domestic audiences—and the Nihon Shoki as outward-facing, meant for international communication. However, this viewpoint has only become clearly articulated through modern research. It remains uncertain whether the compilers in the early 8th century actually had such distinct intentions. Examined carefully, Japanese history reveals that both the Kojiki and the Nihon Shoki were actively utilized within Japan itself over subsequent periods. In other words, Japan’s historical development can be viewed distinctly through the evolving influences of both texts.

The simplistic interpretations arose largely from the specific historical context of their creation. Following the introduction of Buddhism in the mid-6th century, political power struggles intensified, centered primarily around Yamato (present-day southern Nara Prefecture). The imperial court became sharply divided between anti-Buddhist (排仏派, “haibutsu-ha”) and pro-Buddhist (崇仏派, “subutsu-ha”) factions. The anti-Buddhist faction, led by the Mononobe clan—a lineage closely associated with traditional Shinto rituals dating back to the legendary first emperor, Emperor Jimmu—effectively established a monopoly of power by eliminating rivals from the court. Exploiting this internal conflict, the Soga clan quickly expanded its influence, dramatically increasing its political voice.

The exact origins of the Soga clan remain unclear even today, but considering that many people who had crossed over from the continent and the Korean peninsula lived in southern Nara Prefecture at the time, and that the Soga rose through their ties, it is plausible, from my perspective, that the Soga themselves had deep continental connections. With those ties, the Soga had a natural affinity for Buddhism and pushed its adoption onto the emperor to consolidate their influence. The Mononobe, however, opposed this fiercely, arguing that adopting Buddhism—a foreign religion—would incur the wrath of the native Shinto deities (“kami”). With intense daily debates between the factions, the emperor, politically constrained by powerful clans, remained indecisive, inadvertently paving the way for the Soga’s rise to prominence.

The escalating tensions between these anti-Buddhist and pro-Buddhist groups eventually erupted into war, resulting in a decisive victory for the Soga, who were also tied to those who had come from across the sea, the annihilation of the Mononobe clan, and the establishment of Soga dominance over the court. This dictatorship persisted for nearly half a century, fostering deep resentment among descendants of older clans. As Soga supremacy began to crumble, descendants of the Nakatomi clan—historically aligned closely with the Mononobe—and Prince Naka no Oe (中大兄皇子, later Emperor Tenji) staged an armed uprising known as the “Taika Reform” (大化の改新, 645), resulting in a coup d’état. The Soga clan was thus eliminated.

Nevertheless, Buddhism had already rapidly spread across Japan under Soga rule, leaving a profound legacy of temples and statues that even the victors could not escape. Subsequently, Prince Naka no Oe ascended as Emperor Tenji (天智天皇, 668–672) and sought to reclaim governing authority from powerful clans. However, just as he began these internal reforms, significant developments arose on the Korean Peninsula.

In the 7th century, the Korean peninsula was characterized by intense rivalry among three kingdoms—Goguryeo (高句麗), Baekje (百済), and Silla (新羅). Among these, Baekje had historically maintained a deep alliance with Japan, serving as a significant source of cultural and technological transmission. However, in 660, Baekje was conquered by a joint military offensive launched by the Tang dynasty (唐)—the greatest power in East Asia at the time—and Silla. Following Baekje’s collapse, a faction of its royal family and surviving officials fled to Japan, formally requesting assistance in restoring Baekje’s independence, which drastically altered Japan’s internal situation. Emperor Tenji perceived the destruction of Baekje—a key diplomatic and military ally—as a direct threat to Japan’s own national security. He reasoned that if Tang and Silla fully dominated the Korean peninsula, Japan would inevitably become their next target. In response, Tenji dispatched a large-scale expeditionary force to assist in the restoration of Baekje. However, considering Emperor Tenji’s own accession effectively resulted from a coup, it’s clear that Japan entered this foreign war hastily, without proper domestic consolidation or adequate preparation.

Consequently, in 663, the allied forces of Japan and Baekje clashed with Tang and Silla at the Battle of Baekgang (known in Japanese as “白村江の戦い”), located near the mouth of the Geum River in modern-day South Korea. Primarily fought at sea, this battle saw the disorganized Japan-Baekje coalition crushed decisively by the superior tactics and military strength of Tang forces. Historical records indicate massive casualties among Japanese troops, with many soldiers drowning and the fleet utterly decimated. This catastrophic defeat terrified the emperor and the noble clans, convincing them that Japan would soon face invasion. Stricken with panic and abandoning all resolve, the emperor shockingly deserted Yamato—the central region of political authority—and relocated the capital near Lake Biwa in Shiga Prefecture, a more defensible location. This action profoundly disillusioned the people of Yamato, Japan’s political heartland, leading to a rapid erosion of imperial authority. This weakening of central authority directly influenced the subsequent creation of the Kojiki and Nihon Shoki.

The defeat at Baekgang significantly transformed Japanese society, comparable in magnitude to Japan’s modernization in the late Edo period following the Meiji Restoration. Here are the crucial changes briefly outlined:

• Strengthened defensive measures: Japan fortified northern Kyushu with defensive structures such as Mizuki (水城, “Water Fortress”) and Ono Castle to prepare against potential invasions from China and Silla, although these forces were largely gathered from many regions and are said to have lacked allegiance to the emperor.
• Reorganization of state institutions: With diminished central authority, Japan accelerated the development of a centralized governing system through the establishment of the Ritsuryo legal code (律令制), reinforcing the emperor’s political power.
• Reconstruction of diplomatic relations with Tang: Eventually, Japan shifted its approach to avoid direct conflict with Tang, adopting tributary diplomacy through official envoys (遣唐使), which facilitated Buddhism’s widespread flourishing across Japan.

Yet, Emperor Tenji’s retreat disillusioned more than just Yamato’s clans and populace. His behavior was widely condemned as emblematic of the worst kind of ruler, provoking intense anxiety that imperial authority might collapse altogether. Amidst this turmoil, a figure quietly resolved to restore imperial sovereignty from clan dominance and rebuild the nation’s framework. This figure was Emperor Tenji’s younger brother, Prince Oama (大海人皇子).

When Emperor Tenji became gravely ill at the Omi Otsu Palace (近江大津宮) in Shiga, he reportedly summoned Prince Oama to designate him as successor—though this was actually part of a scheme intended to secure the throne for Tenji’s own son, Prince Otomo (大友皇子). By this point, some advisors close to Tenji, recognizing his diminishing authority and foreseeing a more decisive leadership emerging with Prince Oama, secretly warned Oama that accepting the succession would lead to his assassination. Understanding this advice, Prince Oama refused Tenji’s offer, pretending to endorse Prince Otomo’s claim to the throne to evade assassination, and outwardly signaling his lack of imperial ambition by retreating under the guise of becoming a monk to Yoshino Mountain in southern Nara Prefecture. At Yoshino, Prince Oama was covertly supported by En no Ozunu (“En no Gyoja”), a legendary figure known for exceptional wisdom, magical powers, and foresight. Consulting En no Ozunu, who freely traversed the mountainous regions, Prince Oama secretly mobilized tribal groups hiding in the mountains around Omi Otsu Palace, carefully preparing an offensive immediately upon Emperor Tenji’s death.

On January 7, 672, Emperor Tenji passed away, leaving the throne officially to Prince Otomo. However, having meticulously prepared, Prince Oama rapidly mobilized his forces roughly half a year later, on June 24. Within merely a month, Prince Oama decisively defeated Prince Otomo. Returning victoriously to Yamato, he ascended the throne as Emperor Tenmu (天武天皇). Although detailed accounts of the circumstances are lacking, it is reasonable to believe that the clans and people of Yamato enthusiastically welcomed the restoration of strong imperial authority.

Emperor Tenmu immediately launched numerous measures to consolidate imperial power, foremost among which was the “reconstruction of legitimacy.” Reacting against earlier periods when emperors had served merely as figureheads masking real clan authority, this policy aimed to place genuine political control firmly in imperial hands. A crucial obstacle Tenmu faced was the existence of ancient myths and traditions handed down by powerful clans, as these stories legitimized the clans’ historical roots and justified their autonomy. Without clearly connecting these clan traditions to imperial origins, there was no compelling reason for clans to accept imperial supremacy. Recognizing governance would remain impossible while such archaic sentiments persisted, Tenmu concluded that solidifying central authority required systematically reorganizing all genealogical and mythical traditions under imperial oversight. Thus began the groundbreaking national project to compile the Kojiki, fundamentally transforming Japan’s political landscape.