By the autumn of 1614, Tokugawa Ieyasu (徳川家康) had decided that Toyotomi Hideyori (豊臣秀頼) and Osaka Castle would have to be eliminated. The legal pretext was a temple bell inscription deemed insulting. The real reason was simpler: two men could not both claim to govern Japan, and Ieyasu intended to be the one who survived. He assembled a besieging army that, by some estimates, eventually numbered over 200,000 men surrounding the castle and its approaches.

Osaka Castle was formidable but not invulnerable. Its western and northern faces were protected by water. Its southern face was not. The southwest corner, in particular, was the castle's soft underbelly — the place where a determined assault could break through. It was to this corner that Sanada Yukimura (真田幸村, also known as Sanada Nobushige) was assigned.

Yukimura came to Osaka after years in exile at Kudoyama (九度山) in Kii Province, part of the settlement following the Battle of Sekigahara (関ヶ原の戦い) in 1600, where he had fought on the Toyotomi side. He was 47 years old, and he had spent those years in effective house arrest. But Toyotomi Hideyori recalled him because of a single episode: in 1600, Yukimura and his father Masayuki (真田昌幸) had held Ueda Castle (上田城) against Tokugawa Hidetada's (徳川秀忠) entire army for two weeks, delaying Hidetada's reinforcement at Sekigahara. That delay may have cost the Toyotomi side the battle. Hideyori was hiring the man who had once made the Tokugawa look foolish.

What Yukimura built at the southwest corner was called the 真田丸 (Sanada-maru) — literally "Sanada's enclosure" or "Sanada's bailey." It was a semi-circular earthwork barbican, detached from the main castle walls, standing alone as an advance fortification. He did not build it because he was ordered to. He built it because he understood that the only way to defend a position against an army of 200,000 was to force that army to come at him one bottleneck at a time.

On December 3 and 4, 1614, the Tokugawa forces directed approximately 30,000 troops against the Sanada-maru in a series of coordinated assaults. Yukimura had roughly 7,000 defenders. The ratio was more than four to one against him. Every assault was thrown back. Yukimura deployed his arquebusiers — groups of 6,000 gunners by some accounts — in disciplined, rotating fire, and when the assault lines wavered, he launched counterattacks that reportedly broke through the Tokugawa siege lines at least three times. Shimazu Tadatsune (島津忠恒), a famed veteran of the Korean campaigns who had seen the best warriors of his generation, called Yukimura "the number one warrior in Japan." This was not flattery. It was the assessment of one professional to another.

By December 18, 1614, all of the Toyotomi outlying forts had fallen, the waterways were controlled, and the castle was fully encircled. But the Sanada-maru held. The Tokugawa could not take it.

A truce was negotiated on January 22, 1615. The peace terms included the filling-in of the castle's outer moats — and the demolition of the Sanada-maru. The earthwork that no army had been able to take was to be razed as a condition of peace. Observers who watched the demolition reportedly wept. They understood what the loss meant: the southwest corner, once protected by Yukimura's genius, was now simply open ground.

The Summer Campaign began that spring. With the Sanada-maru gone and the moats partially filled, Osaka Castle was strategically compromised before the first trumpet sounded. Yukimura fought to the end at the Battle of Tennoji (天王寺の戦い) on June 3, 1615, leading a charge that came close enough to Ieyasu's position to reportedly cause the old general to consider flight. He died that day, 48 years old, on the losing side. The castle fell. The Toyotomi clan was finished. And Sanada Yukimura's name became inseparable from the Japanese concept of the brilliant warrior who holds the line against history — and loses anyway, magnificently.