By the morning of June 18, 1575, Nagashino Castle (長篠城, Nagashino-jō) had been under siege for two days. Takeda Katsuyori (武田勝頼, Takeda Katsuyori), the twenty-nine-year-old daimyo of the Takeda clan, had brought approximately 15,000 warriors into Mikawa Province (三河国, Mikawa no kuni) to destroy a fortress he considered both a military prize and a personal insult. The castle's commander, Okudaira Sadamasa (奥平貞昌, Okudaira Sadamasa), had previously served under the Takeda. When his family switched allegiance to Tokugawa Ieyasu (徳川家康, Tokugawa Ieyasu) in 1573, Katsuyori had not forgotten. Now Sadamasa, just twenty years old, held the castle with roughly 500 men against a force outnumbering him thirty to one.

The fortress stood at the confluence of the Takigawa (瀧川, Takigawa) and Onogawa (乙川, Onogawa) rivers, making it naturally defensible but also difficult to resupply or reinforce. Katsuyori brought his siege with unusual technical sophistication. He deployed gold miners from Kai Province (甲斐国, Kai no kuni) — men expert in tunneling through rock — to dig under the walls. He floated assault troops across the rivers on rafts. He erected siege towers to threaten the ramparts. The defenders answered each tactic: they countermined, digging their own tunnels to intercept and collapse the attackers' shafts; they flooded approach routes; they repelled every assault from the walls. But they were burning through their rice stores, and the Takeda showed no sign of withdrawing.

Two days before June 18, Ieyasu had sent an urgent appeal to his ally Oda Nobunaga (織田信長, Oda Nobunaga), asking him to march to relieve the castle. Nobunaga, headquartered at Gifu Castle (岐阜城, Gifu-jō) more than 60 kilometers away, made his decision immediately. He would come in person with the full Oda army. The wheels of a larger battle were already turning; June 18 was the moment when both the siege and the relief effort were racing toward the same point.

The crisis inside the castle deepened on the night of June 23, when a low-ranking ashigaru (足軽, ashigaru) named Torii Suneemon (鳥居強右衛門, Torii Suneemon) stepped forward. An ashigaru was the lowest rank of warrior — a common foot soldier, not a samurai. Suneemon volunteered to slip through the Takeda siege lines to deliver a plea for relief. He swam the river in darkness, threaded through enemy encampments, and ran 35 kilometers across mountainous terrain to Okazaki Castle (岡崎城, Okazaki-jō), where he found Ieyasu. Ieyasu pledged to send help immediately. Suneemon agreed to carry the news back to the garrison, turned around, and ran back the way he had come. On his return approach, Takeda soldiers captured him.

The Takeda saw an opportunity. On June 24, they brought their prisoner before the castle walls and made their offer: shout that no relief was coming, call on the garrison to surrender, and your life will be spared. Suneemon agreed — then, facing the walls and the men who were watching from them, shouted something entirely different. He told the defenders that Ieyasu's men were coming. That they should hold on just a little longer. He was killed on the spot, reportedly run through with a spear while still bound to a cruciform post. The garrison heard him. They held.

The story of Suneemon's defiance became one of the most celebrated moments of the Sengoku period (戦国時代, Sengoku jidai). It was remarkable partly because of who he was — not a famous samurai or a general, but the lowest rank of warrior, a man with nothing except his loyalty and a choice about how to use it. A Takeda commander who witnessed the execution was so struck that he adopted the image of the crucified defiant soldier as his own battle standard.

Nobunaga left Gifu on June 21 with approximately 30,000 troops and joined Ieyasu at Okazaki on June 22, bringing the combined relief force to roughly 38,000 men. Experienced Takeda commanders urged Katsuyori to withdraw before this overwhelming force arrived — to preserve the army and fight another day. Katsuyori refused. The Takeda cavalry, riding beneath the banner reading 風林火山 (Fūrinkazan, "Wind, Forest, Fire, Mountain") — the famous motto from Sun Tzu's Art of War — were considered the most feared mounted force in Japan. Their general could not be seen retreating from a smaller castle.

On June 28, the Battle of Nagashino (長篠の戦い, Nagashino no tatakai) was fought on the plain of Shitaragahara (設楽原, Shitaragahara). Nobunaga deployed approximately 3,000 arquebusiers behind wooden palisades in rotating volley fire — a disciplined application of matchlock technology that stopped the Takeda cavalry charges with sustained, overlapping volleys. By the end of the day, the Takeda had lost approximately 10,000 men, including 54 of their 97 named samurai commanders — among them eight of the famous Twenty-Four Generals of Takeda Shingen (武田信玄, Takeda Shingen). The Takeda officer corps was effectively annihilated. The most feared cavalry force in Japan had been stopped not by a better horseman but by peasant foot soldiers with matchlocks and a wooden fence.

The garrison of Nagashino held for twelve days against a 30-to-1 numerical disadvantage. Their twenty-year-old commander, Okudaira Sadamasa, so impressed Nobunaga that the warlord honored him by granting the character "Nobu" from his own name — Sadamasa was thereafter known as Nobumasa (信昌, Nobumasa). As for Katsuyori, the Takeda clan never recovered. He spent seven years in military decline and died by seppuku in 1582 when his retainer base abandoned him. The era of the Takeda cavalry had ended ten days after June 18, on a field in Mikawa.