Jinshin War

The Jinshin War of 672 CE was the largest and most consequential civil conflict in ancient Japan, fought over imperial succession between Prince Ōama — the younger brother of Emperor Tenji — and Prince Ōtomo, the emperor's son. Though lasting barely two months, its outcomes shaped Japanese civilization for centuries: the centralized Ritsuryō state, the formal adoption of the title "Tennō," and the systematic organization of warrior culture that would eventually produce the Japanese sword tradition.

The conflict's origins lay in Emperor Tenji's ambiguous succession arrangements. Tenji had appointed his son Ōtomo as Grand Minister, signaling succession through the main line, but his brother Ōama remained a powerful and popular alternative claimant. When Tenji died in 671, the succession crisis became acute. Ōama had formally entered Buddhist vows and withdrawn to Yoshino — an apparent renunciation — but this withdrawal was tactical rather than sincere. In mid-672, he emerged from Yoshino, declared his intention to fight, and began mobilizing.

Ōama's strategic genius in the Jinshin War is striking for the period. His key move was securing Fuwa barrier in Mino Province, cutting the road between the eastern provinces and the Ōmi capital at Ōtsu. By controlling this chokepoint, he prevented Ōtomo's government from reinforcing or resupplying from the east, while simultaneously consolidating the warrior clans of Mino, Owari, and Ise behind his cause. The Ōtomo government had formal authority and the apparatus of the state, but Ōama had momentum and the loyalty of the provincial military clans.

The war's relevance to sword history lies partly in the military technology of the era and partly in what the war's outcome enabled. The weapons of 672 were straight-bladed tachi (chokutu) and spears — the curved Japanese sword had not yet developed from its Chinese and Korean prototypes. But the iron-working traditions of Mino and Ōwari that supported Ōama's forces were the same regional metalworking cultures that would produce, eight centuries later, the great Mino school of sword-smithing — Kanemoto, Kanesada, and the practical Seki blades that armed the Sengoku armies.

Emperor Tenmu's subsequent reign (673–686) was the period in which Japan's Ritsuryō legal code took shape, the imperial system was systematized, and the sword began to function as an explicit symbol of imperial legitimacy. The imperial regalia — including the sacred sword Kusanagi-no-Tsurugi (Ama-no-Murakumo-no-Tsurugi) — played a formal role in legitimizing the succession Tenmu had won by force. This linking of sword-possession to political legitimacy is a thread that runs from the Jinshin War through the Genpei conflict, the Sengoku era's famous named blades, and into the modern period. The Jinshin War is where that thread begins.