Battle of Mizushima

The Yashima Campaign and the March to Mizushima

In the 7th month of 1183 (Juei 2), the Taira clan, driven from the capital by the forces of Minamoto no Yoshinaka, retreated westward and established a base at Yashima in Sanuki Province. That same year, Yoshinaka entered the capital and secured a position roughly equivalent to shogun, but his relationship with the retired Emperor Go-Shirakawa deteriorated rapidly even as Taira strength steadily recovered along the Inland Sea coast. Alarmed by this, Yoshinaka dispatched a punitive force to strike at the Taira in Yashima, placing it under the command of his trusted retainers, the brothers Ashikaga Yoshikiyo and Yoshinaga, together with Kaino Yukihiro of the Shinano Genji.

This force, numbering some 7,000 mounted men and over 500 ships, pushed westward through the Inland Sea aiming to cross to Shikoku. Most of the Genji troops, however, were warriors from Shinano and Hokuriku accustomed to land warfare, with little experience at sea. Before crossing to Shikoku, they put in at Mizushima in Bitchū Province (present-day Tamashima area of Kurashiki City, Okayama Prefecture), where they encountered a large Taira fleet of over 1,000 ships commanded by Taira no Shigehira, Taira no Tomomori, and Taira no Michimori. On the 1st day of the intercalary 10th month, the two forces suddenly found themselves in battle.

The Annular Eclipse and the Turn of the Naval Battle

The fighting at Mizushima opened with an exchange of arrow fire between the two sides. The Taira, who commanded many experienced Inland Sea naval crews, are said to have lashed their ships together and laid planks across them, constructing a stable fighting platform on the water much like level ground — a tactic reflecting their skill in naval warfare. The Genji, struggling in unfamiliar shipboard combat, then faced a sudden further shock around noon, when the sky abruptly darkened. Records such as the Gyokuyō diary and the Genpei Seisuiki describe this as an annular solar eclipse observed in the region on that day, with the sun greatly obscured and the surroundings falling into a twilight-like gloom.

The Genji troops, largely unfamiliar with astronomical phenomena, are said to have panicked at this sudden change, losing morale and cohesion. The Taira side, by contrast, maintained close ties to the court society versed in calendrical and yin-yang divination knowledge, and a long-held account holds that they had advance knowledge that an eclipse was imminent. Seizing on the Genji's confusion, Taira warriors landed their horses from the ships one after another and threw themselves into close combat, overwhelming the disordered Genji force. In this phase, the Genji commanders — the brothers Ashikaga Yoshikiyo and Yoshinaga, and Kaino Yukihiro of the Shinano Genji — were killed one after another, and the punitive force collapsed entirely, losing its chain of command.

The Aftermath and Yoshinaka's Isolation

Word of the crushing defeat at Mizushima soon reached Yoshinaka in the capital. The failure of the Yashima campaign preserved the Taira's base of power in the west, and fighting along the Inland Sea coast would continue intermittently thereafter. More seriously for Yoshinaka, the defeat dealt a direct blow to his military prestige at a moment when his standing with Go-Shirakawa had already reached a breaking point. In the 11th month of that year, Yoshinaka resorted to the drastic measure of attacking the Hōjūjidono and placing Go-Shirakawa under confinement — an act that provoked backlash across the provinces and hastened his downfall. By the 1st month of 1184 (Juei 3), Yoshinaka was killed at Awazu in Ōmi Province by the forces of Minamoto no Noriyori and Yoshitsune, sent by Minamoto no Yoritomo.

The Battle of Mizushima is remembered as a rare case in which an astronomical event — a solar eclipse — decided the outcome of a battle, and it is frequently cited in the history of astronomy and calendrical science in Japan. At the same time, it exposed the tactical gulf between the eastern and Hokuriku warrior bands, skilled in land combat, and the Inland Sea warrior forces steeped in naval operations. The Tamashima area of present-day Kurashiki City still preserves sites and memorial markers associated with the battle, forming a valuable historical trace of naval warfare during the Genpei War. From the perspective of arms and armor history, the Taira navy's tactic of lashing ships together is understood as a formation designed to give an advantage in close combat fought with tachi swords and naginata, offering a glimpse of how blades were deployed in late Heian-era naval fighting.