The logs went into the river unmanned. No soldiers stormed the base of the cliff. No siege towers creaked forward across open ground. The weapon was the current itself.

二俣城 (Futamata-jo) — Futamata Castle — occupied a long, narrow ridge at the confluence of the 天竜川 (Tenryugawa), the Tenryu River, and the Futamata River in Totomi Province (present-day Hamamatsu, Shizuoka). Tokugawa Ieyasu had garrisoned it as a forward position guarding the road into his home territory. The castle's defenders held what appeared to be an unassailable position: sheer cliffs on multiple sides, a narrow neck of land to the north as the only approach, and a water supply drawn by means of a timber well-tower built down the cliff face directly to the river below. As long as that tower stood, the garrison could hold indefinitely.

Takeda Shingen began his great western campaign in the autumn of 1572 with approximately 22,000 men — the legendary 武田軍 (Takeda-gun), Takeda army, whose cavalry had made the clan the most feared military force in Japan. Futamata was the first obstacle. Shingen assigned direct command of the siege to his son and heir, 武田勝頼 (Takeda Katsuyori), then 25 or 26 years old.

A frontal assault up the cliff was not a serious option. Katsuyori's engineers found another way.

They cut timber and lashed it into heavy rafts. They carried the rafts upstream and launched them into the Tenryu's current. Unmanned, the rafts drifted downstream and struck the wooden support pillars of the well-tower again and again — not a dramatic explosion of force but a patient, methodical hammering. Timber cracks slowly. Supports weaken. Eventually the well-tower groaned and toppled into the water it had once made accessible. The garrison watched their lifeline fall into a river they could no longer reach.

The siege had opened in late October or November 1572. The garrison held on for two months — without water there is no other option but slow capitulation. They surrendered in late December (January 22, 1573 by the Gregorian calendar).

The castle fell without a direct assault on the walls. Not a single ladder had gone up the cliff face.

Shingen immediately pressed the advantage. He advanced toward Hamamatsu, drawing Ieyasu out for a battle the younger lord could not avoid. At 三方ヶ原 (Mikatagahara) in January 1573, Ieyasu's army was routed so thoroughly that he reportedly soiled himself in the retreat — a story that, whether true or apocryphal, says everything about the scale of the defeat. Ieyasu survived by fleeing back to Hamamatsu Castle. He later ordered a self-portrait painted at his most humiliated, to remind himself never to repeat the error.

Had Shingen lived, the Tokugawa clan might not have survived him. But Shingen died of illness on May 13, 1573, before he could press his advantage to the capital. The great campaign died with him.

Katsuyori inherited the clan and tried to continue his father's work. He lacked Shingen's patience. In 1575, at the Battle of 長篠 (Nagashino), he threw the Takeda cavalry — the pride of the clan — into Oda Nobunaga's (Oda Nobunaga's) arquebus lines in wave after wave of frontal assault. The cavalry was shattered. The Takeda were finished.

The raft gambit at Futamata had been Shingen's last great display of the quality that made him dangerous: the willingness to find the indirect solution, to attack not the wall but the dependency behind it. His enemies could armor their walls. They could not armor their water supply.

Futamata Castle later passed back to Tokugawa hands after the Takeda collapse in 1575. It became the site of one of Ieyasu's darkest acts: in 1579, his own son Nobuyasu was imprisoned at Futamata and forced to commit 切腹 (seppuku) there, at Oda Nobunaga's insistence. The castle that had taught Ieyasu one lesson in humility became the place where he would suffer another.