What was the Odawara Campaign?
The Odawara campaign was the last act of the unification. Toyotomi Hideyoshi brought an overwhelming army against the Later Hojo, who had governed the Kanto plain from Odawara for a century, and starved them out in about three months.
It was less a battle than a demonstration. The point was to show every remaining lord in Japan what defying the new order would cost.
Why did it happen?
Hideyoshi had issued a general prohibition on private warfare among daimyo. In 1589 a Hojo retainer seized Nagurumi Castle, which had been assigned to the Sanada by Hideyoshi's own arbitration. That was the pretext, and the Hojo refusal to pay court in person was the substance.
The Hojo believed Odawara could not be taken. They had held it against Uesugi Kenshin in 1561 and Takeda Shingen in 1569, and had extended its outer defensive line to enclose the entire castle town.
Forces and sides
Hideyoshi's army is usually given as around 200,000 to 220,000, against perhaps 50,000 in Odawara and its satellite castles. Even allowing for exaggeration, the disparity was enormous.
Just as important was the naval and logistical effort: Hideyoshi controlled the sea approaches and could feed his army for months, which is what made a passive siege possible.
What happened?
The Toyotomi army moved along two routes, reducing the outlying Hojo castles as it came — including Hachioji, taken by storm with heavy casualties, and Oshi, where a water attack failed.
Odawara itself was surrounded and simply invested. Hideyoshi made the siege deliberately theatrical: he built Ishigakiyama Castle on a hill overlooking the town — the traditional story is that it was revealed all at once by cutting away the screening trees, so that the defenders woke to a completed castle where there had been forest — and brought in tea masters, entertainers and the families of his commanders.
Inside the castle, the Hojo leadership held councils that reached no decision for weeks. The Japanese phrase "Odawara conference" still means a discussion that goes nowhere.
The castle surrendered in early August 1590.
Turning point
There was no combat turning point. The campaign turned on the fact that Hideyoshi could keep 200,000 men supplied indefinitely and the Hojo could not out-wait him — a demonstration of the difference between a national state and a regional one.
Outcome
Hojo Ujimasa was ordered to commit suicide; his son Ujinao was exiled and died shortly afterwards. The house that had ruled the Kanto for a century ended.
Ieyasu was transferred out of his ancestral provinces into the newly taken Kanto, and chose Edo — then a minor castle town — as his base. That decision produced modern Tokyo.
Date Masamune and the remaining northern lords submitted in the weeks that followed. The unification was complete.
Why it mattered
Odawara ends the era of independent regional warfare that began with the Onin War 123 years earlier. After 1590 there was one authority in Japan, and the only question was who would hold it.
It also created the conditions for what came next: with no more territory to conquer at home, Hideyoshi turned outward, to Korea. See Japanese Invasions of Korea (1592–1598).
