When Toyotomi Hideyoshi (豊臣秀吉, Toyotomi Hideyoshi) died in September 1598, he left behind a Japan that was unified in name but not in power. He had extracted promises from his five senior administrators, the bugyo (奉行, bugyo), and five great military lords to protect his young heir, Toyotomi Hideyori (豊臣秀頼, Toyotomi Hideyori), until the boy came of age. Tokugawa Ieyasu (徳川家康, Tokugawa Ieyasu) had sworn the same oath. Within months, Ieyasu was arranging unauthorized marriage alliances among daimyo and building military blocs in clear violation of his pledge. Hideyoshi had been dead barely a season.

Of the five bugyo, Ishida Mitsunari (石田三成, Ishida Mitsunari) was the most vocal in opposing Ieyasu's maneuvering. Mitsunari was 41 in 1600, born in 1559, and had served Hideyoshi as a bureaucratic genius for most of his adult life — managing logistics, administering territories, organizing the vast machinery of Hideyoshi's government. He was not a battlefield commander. He was precisely the kind of man that warrior-daimyo despised: clever, meticulous, and contemptuous of blunt force as a political tool.

In early 1599, that contempt caught up with him. Seven senior military commanders, led by Fukushima Masanori (福島正則, Fukushima Masanori) and others who nursed deep grudges against him, surrounded his mansion with intent to kill. Mitsunari fled to Ieyasu's compound — an extraordinary irony — and Ieyasu, calculating that a live enemy was more useful than a martyred one, protected him. The price of protection was Mitsunari's resignation from office and retirement to Sawayama Castle (佐和山城, Sawayama-jo) in Omi Province (近江国, Omi no Kuni). By spring 1599, Japan's sharpest political mind had been removed from the board.

But in late May 1600, Ieyasu made a move that reopened the game. He confronted Uesugi Kagekatsu (上杉景勝, Uesugi Kagekatsu) of Aizu over reports of military fortification, demanding an explanation. Kagekatsu, who had been corresponding secretly with Mitsunari for months, refused around May 7, 1600. Ieyasu now had a pretext to march east with his army to punish the insubordinate Uesugi — and in doing so, he would pull his forces away from Osaka and the heartland of Japan. Mitsunari had been waiting for exactly this.

From Sawayama, without title or official standing, Mitsunari began building his coalition. The work was done entirely through correspondence — letters addressed to the great daimyo of western Japan, each carefully tailored to its recipient's particular ambitions and grievances. He wrote to the Shimazu (島津, Shimazu) of Satsuma. He wrote to the Chosokabe (長宗我部, Chosokabe) of Shikoku. He maneuvered to secure the backing of the Mori (毛利, Mori) clan, the most powerful in western Japan, whose head Mori Terumoto (毛利輝元, Mori Terumoto) would eventually become the nominal commander of the Western Army. Mitsunari's letters from this period survive, and they reveal a mind operating at full intensity — addressing territorial disputes, promising redress of old injustices, invoking the memory of Hideyoshi and the obligation to protect Hideyori.

The central challenge was personal. Mitsunari was an administrator trying to raise an army from men who regarded administrators with contempt. He could not inspire loyalty through martial reputation. He could only appeal to interest and principle — and hope that enough daimyo calculated that Ieyasu's rise was more dangerous to them than the risks of open war. Enough of them did.

By August 16-17, 1600, formal coalition meetings at Sawayama confirmed the Western Army's structure, with Terumoto as figurehead and Mitsunari as its operational mind. On August 22, the coalition formally denounced Ieyasu. On October 21, 1600, the two armies met at Sekigahara (関ヶ原の戦い, Sekigahara no Tatakai). The Western Army was larger on paper. But when Kobayakawa Hideaki (小早川秀秋, Kobayakawa Hideaki) defected mid-battle — his loyalty having been secured by Ieyasu's promises — the coalition that Mitsunari had spent months building collapsed in hours. Mori Terumoto, the nominal commander, never left Osaka Castle. The Western Army fought without its general and fell apart when its center turned.

Mitsunari fled into the hills. He was recognized by farmers and turned over to Ieyasu's men — a bitter end for a man who had claimed to champion Hideyoshi's legacy among the common people. He was beheaded in Kyoto on November 6, 1600. He was 41 years old. His plan had been geopolitically sophisticated. It failed not in conception but in the loyalty of the men he needed most, who made different calculations on the day of battle.