Somewhere upstream on the Ane River, Azai Nagamasa was fighting his way toward his sister's brother. And somewhere downstream, Oda Nobunaga was fighting his way toward his sister's husband. The river was ankle-deep and wide, the summer morning already brutal with heat, and approximately 46,000 men had turned its muddy banks into a killing ground.

The alliance between Nobunaga and Nagamasa had been sealed in 1564 with a marriage — Nobunaga's younger sister Oichi sent north to Odani Castle as Nagamasa's bride. She was beautiful. He was ambitious. The match was also a living contract, a human bond binding two clans together. When Nobunaga marched against the Asakura clan of Echizen that spring, Nagamasa faced the choice that no man in the 戦国時代 (Sengoku jidai) era could avoid indefinitely: which obligation mattered more? His brother-in-law's alliance, or his father's old ties to the Asakura?

He chose the Asakura. He was 25 years old and he had principles. In this era, that was roughly equivalent to having a death wish.

There is a famous story — beloved, unverifiable — that before the betrayal became open war, Oichi herself sent Nobunaga a warning. A small bag of beans, tied shut at both ends. A coded message: you are about to be trapped from both sides, north and south. Nobunaga, the legend says, understood. He retreated in time. Then he came back with an army.

On July 30, 1570, that army met the combined Azai-Asakura forces on the plain of the 姉川 (Anegawa) in Ōmi Province 近江国 (Ōmi no kuni). Nobunaga commanded the right flank against the Azai. His junior ally Tokugawa Ieyasu, 27 years old, held the left against the Asakura. Among the Oda commanders wading into the shallow water that morning for his first open engagement: Hashiba Hideyoshi, 33, the son of a low-ranking foot soldier who would one day rule all of Japan.

The three men who would unify the country were all in this river.

The fighting was grinding and close. The Ane River in summer offers nowhere to run — men stood knee-deep in the current, grappling with spears and swords while their footing shifted in the mud. Nobunaga had deployed approximately 500 arquebusiers 鉄砲足軽 (teppō ashigaru) — an early use of massed firearms in Japanese field combat, five years before the famous tactics at Nagashino. Even with gunpowder, the battle lasted hours.

The allied forces won. Perhaps 1,700 Azai and Asakura men died; perhaps 800 of Nobunaga's and Ieyasu's. Nagamasa retreated to the mountain fortress of Odani Castle. The Asakura withdrew to Echizen. Neither clan was destroyed. Oichi watched from the castle walls above the valley, knowing her husband had failed and knowing her brother had not yet finished.

The 裏切り (uragiri) — the betrayal — had been committed. The battle was won. The reckoning was three years away.