By the spring of 1590, Toyotomi Hideyoshi (豊臣秀吉, Toyotomi Hideyoshi) had unified most of Japan through a combination of military force, strategic marriage, and overwhelming logistical power. One clan still held out: the Hojo (ほうじょうけ) of Odawara. The later Hojo — also called Go-Hojo (後北条, Go-Hōjō) to distinguish them from the earlier regent family — had ruled the Kanto plain for five generations, since roughly 1495. Their seat, Odawara Castle (小田原城, Odawara-jō), was not merely a fortress. Its walls enclosed an entire city. It had withstood sieges by two of the era's greatest commanders — Takeda Shingen (武田信玄, Takeda Shingen) in 1561 and Uesugi Kenshin (上杉謙信, Uesugi Kenshin) in 1561 — and was regarded as impregnable.

When Hideyoshi demanded submission in early 1590, the Hojo refused. He mobilized a force estimated at approximately 200,000 men and marched on Odawara. It was the largest army Japan had ever assembled. He arrived in late spring and established a siege camp around the castle — but what he did next was not what any general had done before.

He turned the siege into a performance of power. Hideyoshi invited nobles, wives, concubines, entertainers, and tea masters to join him in the camp. He held formal tea ceremonies. He organized moon-viewing parties. He wrote poetry. The message to the Hojo, and to every daimyo (大名, daimyō) watching from the surrounding domains, was deliberately maddening: I have already won. I am not even trying. The Hojo council met endlessly to debate whether to fight, negotiate, or surrender — and made no decision. That paralysis gave rise to the phrase 小田原評定 (Odawara hyōjō) — the Odawara council — which passed into the Japanese language as a proverb for deliberation so prolonged that the situation resolves itself without a decision ever being reached. It is still used today.

While the Hojo deliberated and Hideyoshi performed, a second project was underway in secret. On Kasagakeyama (笠掛山, Kasagakeyama) hill, approximately three kilometers west of Odawara Castle — in plain sight of the fortress's walls — roughly 40,000 workers began constructing a castle. The existing forest screened the work from Hojo watchers. For approximately 80 days, stonemasons, carpenters, and laborers raised stone walls, towers, and gates on the hilltop. The Hojo defenders saw laborers moving in the trees. They did not know what they were building.

When the construction was complete, Hideyoshi gave the order. In a single night, the trees screening the castle were felled. At dawn, the Hojo defenders on Odawara's walls looked west and saw something that should not have been there: a fully formed castle, stone walls gleaming in the morning light, towers rising against the sky, banners flying. It had appeared overnight.

The psychological effect was crushing. The defenders now understood what Hideyoshi had been doing while they debated. He had the resources to build a castle in secret while hosting parties in his camp. He had the patience to wait for eighty days. He had more men than they could count, and he hadn't bothered to use most of them. The message was no longer implicit. The Hojo's famous walls enclosed a city — and a fully armed castle now looked down at those walls from three kilometers away.

The castle was named 石垣山一夜城 (Ishigakiyama Ichiya-jō) — Stone Wall Mountain One-Night Castle. The name was propaganda preserved in stone: ichiya (一夜, ichiya) means 'one night.' The castle was not built in one night, but it appeared in one night, and that was enough.

The Hojo surrendered that summer. Hojo Ujimasa (北条氏政, Hōjō Ujimasa), the clan head, was 52 years old. He was ordered by Hideyoshi to commit seppuku (切腹, seppuku), which he did. His son Ujinao (氏直, Ujinao) was spared — he was married to Tokugawa Ieyasu's (とくがわいえやす) daughter, and Ieyasu intervened on his behalf. Ujinao was exiled to Kōyasan (高野山, Kōyasan) and died there the following year.

With the Hojo gone, Hideyoshi redistributed their vast Kanto holdings. The lands went to Tokugawa Ieyasu as a kind of prize — and a kind of exile, moving him far from the center of power. Ieyasu accepted, relocated to Edo (江戸, Edo, present-day Tokyo), and began building. He could not have known yet that Hideyoshi had just handed him the base from which his descendants would rule Japan for 250 years.