Battle of Idano

What Happened

On 5 December 1535, the forces of the Matsudaira clan (松平氏, Matsudaira-shi) met the army of the rebel vassal Abe Masatoyo (阿部正豊, Abe Masatoyo) in battle at Idano, in Mikawa Province (三河国, Mikawa no kuni). The Matsudaira were victorious. The engagement was fought as an act of revenge, seven days after Abe Masatoyo had killed the Matsudaira leader Matsudaira Kiyoyasu (松平清康, Matsudaira Kiyoyasu).

The battle took place during the Tenbun era (天文, Tenbun), in its fourth year — 1535 by the Western calendar. Idano lies in the part of Mikawa that corresponds to the modern prefecture of Aichi, at roughly 34.98° north, 137.17° east.

Background

Matsudaira Kiyoyasu was the grandfather of Tokugawa Ieyasu (徳川家康, Tokugawa Ieyasu), the man who would later unify Japan and found the Tokugawa shogunate. In the early 1530s Kiyoyasu was the rising power of Mikawa Province, expanding his clan's holdings at the expense of neighbouring lords. His own vassal, Abe Masatoyo, killed him — a murder within the clan's own following rather than a death in open battle against a rival house.

The killing left the Matsudaira leaderless and the clan's retainers facing a double problem: an external enemy would certainly test the clan's strength, and the murderer himself was still at large with an army. The response came within days. Seven days after the murder, the Matsudaira forces marched out to take revenge on Masatoyo and his army, and destroyed them at Idano.

Why It Mattered

The battle settled the immediate crisis of the murder. The rebel was defeated in the field, and the Matsudaira avenged their lord. The clan survived the killing of its leader, and the line that produced Tokugawa Ieyasu continued.

The battle belongs to the Sengoku period (戦国時代, Sengoku jidai), the century and a half of near-continuous warfare among regional lords that ran from the middle of the fifteenth century into the early seventeenth. It also falls within the Muromachi period (室町時代, Muromachi jidai), the stretch of Japanese history dated 1336 to 1573, when the Ashikaga shoguns held nominal national authority in Kyoto while real power fragmented among provincial warrior houses such as the Matsudaira. Idano is one small episode in that fragmentation: a vassal turning on his lord, and a clan's retainers closing ranks to punish him.

The Name in Japanese

The battle is written 井田野の戦い and read いだののたたかい (Idano no tatakai).

• 井 — read い (i) here. It means "well", as in a water well. It is one of the most common elements in Japanese place names.
• 田 — read だ (da) here. It means "rice field" or "paddy". In place names it very often appears as だ or た.
• 野 — read の (no) here. It means "field", "plain" or "open country" — land outside the cultivated paddies.
• の — the hiragana particle の (no), which links two nouns. Here it joins the place name Idano to the word for battle, in the pattern "X の戦い", "the battle of X".
• 戦 — read たたか (tataka) here. It means "fight" or "war"; the full verb is 戦う (tatakau), "to fight".
• い — the hiragana い (i), the ending of the verb form 戦い (tatakai), "a battle".

Taken together, 井田野 (Idano) is a place name built from three landscape words — well, rice field, open field — a typical formation for a rural locality in Mikawa. The page's source material also gives the name in the alternative form 井田野合戦, using 合戦 (かっせん, kassen), a word for a battle or military engagement that is common in the titles of Sengoku-era clashes.

Key Figures

Matsudaira Kiyoyasu (松平清豊 / 松平清康, Matsudaira Kiyoyasu) — leader of the Matsudaira clan and grandfather of Tokugawa Ieyasu. He was murdered by his vassal Abe Masatoyo shortly before the battle; his death was the grievance the Matsudaira forces set out to avenge.

Abe Masatoyo (阿部正豊, Abe Masatoyo) — the Matsudaira vassal who killed his lord Kiyoyasu. Seven days later he faced the Matsudaira army at Idano and was defeated.

Tokugawa Ieyasu (徳川家康, Tokugawa Ieyasu) — grandson of Kiyoyasu, named here because the battle's outcome preserved the clan into which he was born. He was not present at Idano; the battle belongs to the generation before his rise.