Battle of Sunomata-gawa

What Happened

On 8 June 1181, Minamoto no Yukiie (源行家) attempted a night attack across the Sunomata River against the Taira forces arrayed on the opposite bank. The action took place at Sunomata, near the border between Owari and Mino provinces, on the river known today as the Nagara. Yukiie's plan was an ambush under cover of darkness: his men would wade the river and fall on the Taira camp before it could form up.

The plan failed. The Taira commander Taira no Tomomori (平知盛) had his army waiting directly across the water, and when the Minamoto warriors emerged from the river, the Taira could tell friend from foe even in the dark — the attackers were soaked and dripping, their own men were dry. The ambush collapsed into a rout, and Yukiie and the surviving Minamoto were driven back across the river.

The retreat did not end at the Sunomata. The Minamoto fell back to the Yahagi River in Mikawa Province, and the Taira army pursued them there.

Background

The battle belongs to the Genpei War (源平合戦), the civil war between the Minamoto and Taira clans that ran from 1180 to 1185 and ended with the establishment of the Kamakura shogunate. It was fought in the Heian period, in the first year of the Yōwa era (養和), a reign name adopted in 1181.

Earlier in 1180, Minamoto no Yoritomo had raised an army in the east, and the Taira under Taira no Kiyomori had answered with force. Kiyomori's death in 1181 changed the shape of the war: leadership of the Taira passed to his son Tomomori and others of the clan, and the Minamoto pressed their advantage. Yukiie, Yoritomo's uncle and one of the most persistent Minamoto commanders, was operating in the central provinces, and the Sunomata crossing was his attempt to strike the Taira army in the field.

The site sits at a crossing point between Owari and Mino, on the road network that connected the eastern provinces with the capital region — the kind of ground where two armies moving toward each other were likely to meet.

Why It Mattered

Sunomata-gawa was a Minamoto defeat, and a sharp one: a night attack that depended on surprise was detected and broken, and the survivors were chased from the field to the Yahagi River. It showed that the Taira, despite the loss of Kiyomori the previous year, could still fight a coordinated campaign in the centre of the country.

The battle is remembered less as a turning point than as one engagement in the long attrition of the Genpei War. The Taira pursuit to Mikawa kept the Minamoto off balance in the short term, but the war's momentum lay with the eastern Minamoto, and within four years the Taira were destroyed at Dannoura.

The engagement is also tied in the record to the Mino Genji's raising of an army — the Minamoto of Mino Province taking up arms in the same theatre. Sunomata thus marks a moment when the conflict spread through the provincial warrior houses of central Japan rather than remaining a contest between two national leaders.

The Name in Japanese

The Japanese name is 墨俣川の戦い, read in kana as すのまたがわのたたかい, romanised as Sunomata-gawa no Tatakai.

Broken down character by character:

• 墨 — read here as sumi (すみ). The character means "ink", specifically the black ink used in calligraphy and sumi-e painting.
• 俣 — read here as mata (また). A character meaning "fork" or "branch", used of a place where a stream or road divides. Together, 墨俣 (Sumomata) is the place name, written in the sources as Sunomata.
• 川 — read here as gawa (がわ), the voiced form of kawa. It means "river". When it follows another word, the initial k voices to g, which is why the name is Sunomata-gawa rather than Sunomata-kawa.
• の — the possessive particle, written in kana rather than kanji. It links the river name to the word for battle, roughly "of".
• 戦 — read here as tatakai (たたかい). It means "battle" or "war"; on its own the character is also read sen, as in 戦争 (sensō, war).
• い — kana, completing the reading tatakai.

So the whole name parses as "the battle of Sunomata River": a place name of two characters, the river character, the linking particle, and the noun for battle. The reading Sunomata for 墨俣 is a local place-name reading and not the ordinary reading of either character in isolation — 墨 alone is normally sumi, and 俣 is rare outside place names and surnames, where it usually appears as mata.

Key Figures

Minamoto no Yukiie (源行家) — the Minamoto commander at Sunomata. He led the night crossing of the river and, after the ambush was detected, escaped with the surviving Minamoto back across the water and on to the Yahagi River in Mikawa.

Taira no Tomomori (平知盛) — the Taira commander. His army was drawn up on the far bank of the Sunomata River, opposite Yukiie's force, and it was Tomomori's men who identified the dripping attackers in the darkness and broke the assault. He pursued the retreating Minamoto to the Yahagi.

The Mino Genji — the Minamoto of Mino Province, whose raising of an army is recorded in connection with this engagement, marking the widening of the Genpei War into the provincial warrior networks of central Japan.