徳川家康 (Tokugawa Ieyasu) had been looking for a pretext, and in the autumn of 1614, he found one. A bell cast for the rebuilt 方広寺 (Hōkō-ji) temple in Kyoto bore an inscription — 国家安康 (kokka ankō), "may the state be peaceful and prosperous" — that Ieyasu claimed split his own name, 家康 (Ieyasu), as a deliberate curse. Whether he believed it or not hardly mattered. He needed a reason to move against Osaka Castle, where 豊臣秀頼 (Toyotomi Hideyori) — Hideyoshi's 22-year-old son, the last living symbol of the old order — sat with a garrison filling daily with 浪人 (rōnin) who had no other lord and nothing left to lose.

Ieyasu assembled 164,000 men. It was the largest military mobilization since Sekigahara. The Toyotomi had perhaps 90,000 to 100,000 defenders — many of them veteran fighters, but politically isolated, with no allied daimyo who dared send troops. It was supposed to be overwhelming.

The southwest corner was the problem. Osaka Castle was formidable on most sides, but its southern face opened onto a flat plain — ideal ground for massing an attacking force. Military doctrine and common sense both said: hit the weak point hard. Ieyasu's generals were counting on it.

真田信繁 (Sanada Nobushige) — "Yukimura" the later popular name by which history remembers him — was 47 years old and had spent the last fourteen years at 高野山 (Kōyasan), the mountain monastery where the Tokugawa had sent him after he chose the wrong side at Sekigahara in 1600. He had been sent there to fade away quietly. Instead, when the call came from Osaka, he came down from the mountain.

He looked at the flat ground south of the castle and he built on it.

The 出丸 (demaru) — outwork, forward fortification — that Yukimura constructed became known as the 真田丸 (Sanada-maru): an earthwork barbican extending the castle's defensive perimeter outward onto the very ground the attackers planned to use for their approach. Its garrison was approximately 6,000 men. Its design incorporated arquebusiers firing from elevated earthworks. The flat approach that was supposed to be the easy road into Osaka became instead a field of intersecting fire.

徳川家康 (Tokugawa Ieyasu) began his operations on November 15, 1614, positioning forces in the hills around Osaka. On November 19, he personally led men across the Kizu River, reducing the fort at Kizugawaguchi. Through late November, outlying Toyotomi forts fell one by one. By November 30, the last of the defenders had pulled back inside Osaka Castle and the formal siege began.

The assault on the 真田丸 (Sanada-maru) came on December 3–4. Three of Ieyasu's commanders moved against it simultaneously.

前田利常 (Maeda Toshitsune), 21 years old, commanded 12,000 men — the single largest force in the Tokugawa army, the pride of the Maeda clan, the most powerful noble house outside the Tokugawa. He was repulsed. His force, a contemporary account noted, was completely defeated.

井伊直孝 (Ii Naotaka), 25 years old, led 4,000 men from the Ii clan — the famous 赤備え (akabisoke), the Red Devils, warriors identifiable by their red-lacquered armor, descendants of the soldiers his legendary father 井伊直政 (Ii Naomasa) had led at Sekigahara. They were repulsed.

松平忠直 (Matsudaira Tadanao), 19 years old, Ieyasu's own grandson, brought 10,000 men. He was repulsed.

Twenty-six thousand attackers against 6,000 defenders. The ratio was worse than four to one. Every assault failed.

Yukimura's troops counterattacked. Through the winter campaign, they broke through Tokugawa siege lines three separate times. The enemy's own veterans, watching from the other side of the earthworks, started talking. 島津忠恒 (Shimazu Tadatsune) — a battle-hardened commander who had fought in Korea and Sekigahara both — named Yukimura 日本一の兵 (Nihon'ichi no tsuwamono): the number one warrior in Japan.

Ieyasu, confronted with an embarrassment that 164,000 men had not prepared him for, eventually resorted to tunneling cannon under the castle walls to shake the defenders into a peace negotiation. The Osaka Winter Truce came in January 1615. Under its terms — which Ieyasu interpreted expansively — the outer moat of Osaka Castle was filled in. The castle's defensive advantage was gone. The summer campaign of 1615 destroyed Osaka Castle entirely, extinguished the Toyotomi line, and killed 真田信繁 (Sanada Nobushige) at the Battle of 天王寺 (Tennōji), reportedly in a last personal charge toward Ieyasu's command post.

But that was later. In late November 1614, standing on the flat ground south of the castle, an exile who had been sent away to be forgotten was building the most famous fortification in the history of samurai warfare. He had 6,000 men and all the time the winter would allow.