Battle of Sekigahara 1600: How Betrayal Decided Japan's Future

On the morning of 21 October 1600, thick fog lay over a narrow valley in Mino Province, today's Sekigahara in Gifu Prefecture. It lifted only around 8 a.m., and the first vanguards collided. Between them, the two sides fielded roughly 150,000 to 170,000 men, the largest field battle Japan had ever seen.

And yet, as many accounts see it, it wasn't decided only that morning. Tokugawa Ieyasu didn't win because his troops fought better, but because he knew before the first shot that several commanders on the other side would switch sides or stay inactive.

In short: the Battle of Sekigahara took place on 21 October 1600. The Eastern Army under Tokugawa Ieyasu defeated the Western Army under Ishida Mitsunari, who fought in the name of the young Toyotomi Hideyori, within a few hours, mainly through betrayal inside the Western Army's ranks. Ieyasu became shōgun in 1603, and Tokugawa rule lasted until 1868.

When Toyotomi Hideyoshi died in 1598, his son Hideyori was only five years old. Hideyoshi had made provisions: a Council of Five Elders, made up of Tokugawa Ieyasu, Maeda Toshiie, Mōri Terumoto, Uesugi Kagekatsu, and Ukita Hideie, was to secure Hideyori's succession, while five commissioners, among them Ishida Mitsunari, ran the day-to-day business. Mitsunari therefore was not on the Council of Elders, but was known more for his political than his military skills. Ieyasu was 57 in 1600, Mitsunari in his early forties. Ōtani Yoshitsugu, a close friend of Mitsunari, warned him against the war, because many daimyō would rather side with Ieyasu.

The system lasted only about a year. In 1599, Maeda Toshiie, the only one who could meet Ieyasu as an equal, died, and from then on Ieyasu's power grew. In the summer of 1600, he marched with his "Eastern Army" toward the east after growing suspicious of Uesugi Kagekatsu's military buildup, and Mitsunari in Osaka brought charges against him and raised the "Western Army." Mōri Terumoto, one of the Elders, became nominal commander in chief, but stayed in Osaka Castle.

From 27 August to 8 September, the Western Army besieged Fushimi Castle near Kyoto, which Torii Mototada held for Ieyasu with around 2,000 men against roughly 40,000 attackers. In the end the castle fell, and Mototada was killed. Among the besiegers was Kobayakawa Hideaki, the man who would play a key role at Sekigahara.

For a quick lookup of the sequence, here are the key dates, by the Gregorian calendar. Many sources instead give Japanese lunar-calendar dates, which shift by weeks.

The numbers vary, depending on the source, about 75,000 to 88,000 men stood in the Eastern Army and roughly 80,000 to 85,000 in the Western Army. On paper, the Western Army held the stronger position: Mitsunari's plan called for Ukita, Shimazu, and Ōtani to hold Ieyasu in the valley until Kobayakawa and Mōri came down from the mountains and caught him in a pincer. Ukita Hideie led about 17,000 men, Mitsunari himself about 6,900, Kobayakawa Hideaki on Mount Matsuo about 15,600, and Mōri Hidemoto another roughly 15,000.

Mitsunari had left Ōgaki Castle on the evening of 20 October, knowing Ieyasu had to follow: the road to Osaka and to Mitsunari's headquarters at Sawayama led through the Sekigahara valley. Ieyasu followed from around 2 a.m. and set up his command post at Momokubari next to Nanguyama. His son Hidetada had not yet arrived with around 38,000 men, which, according to one account, made Ieyasu furious. Around 8 a.m., as the fog lifted, Ii Naomasa's shock troops met the forces of Ukita Hideie, and the battle began.

Kobayakawa Hideaki was a close relative of Hideyoshi, had lost territories after the Korean campaign, regained them only after Hideyoshi's death, and afterward deepened his relationship with Ieyasu. A few days before the battle, he sent Ieyasu a message: he would switch to the Eastern Army if rewarded with great wealth and land. Mitsunari heard rumors of treason and made the young commander promise to attack the Tokugawa flank as soon as he lit a signal fire.

Behind the scenes, Kuroda Nagamasa negotiated with Hideaki, Wakisaka Yasuharu, and Kikkawa Hiroie. Kikkawa Hiroie, a vassal of the Mōri, promised the Mōri would switch sides during the battle if they were pardoned after the war. According to the Samurai Museum Berlin, Ieyasu won not through better generalship but through letters, secret meetings, and pledges arranged weeks and months before the day of battle. He knew that Hideaki, Wakisaka, Kikkawa, and at least four other prominent Western commanders would switch sides or stay inactive.

The battle's most famous scene goes like this: Kobayakawa Hideaki sits with his 15,600 men on Mount Matsuoyama and doesn't move, although Mitsunari's signal fire burns around 11 a.m. Ieyasu loses patience and has musketeers fire at Hideaki's position as an ultimatum: decide now. Hideaki decides and attacks Ōtani Yoshitsugu. This version appears in many accounts, including from the US National Park Service, and the Samurai Museum Berlin cites contemporary chronicles for it.

Newer research sees it differently. Wikipedia cites, after Osprey, the assessment that the earliest accounts of Sekigahara show Hideaki's "treachery" occurring not halfway through the battle but at its start, making the story of the warning shots "a complete myth." One summary of newer research considers it very likely a later invention, because records and letters by participants from right after the battle don't mention it. Other accounts note that whether it was provocation or frustration remains debated.

The assessment is therefore more honest than the scene promises: on the newer reading, Hideaki didn't need to be forced, he had long since decided. A shot into the void would be dramatic, but possibly unnecessary for the outcome.

Hideaki's roughly 15,000 men stormed down the slope onto Ōtani Yoshitsugu, whose around 600 men, later reinforced to about 2,400, had anticipated betrayal and adjusted their lines. Against the superior numbers it didn't help. Ōtani, seriously ill according to the sources, took his own life. With the flank gone and further defections, among others by Wakisaka Yasuharu, Kutsuki Mototsuna, Akaza Naoyasu, and Ogawa Suketada, the Western Army collapsed. The Mōri stayed idle because Kikkawa Hiroie blocked the road their roughly 15,000 men would have had to take. The 28,000 Western troops at Nanguyama, according to the National Park Service, did practically nothing all day.

Shimazu Yoshihiro stood idle all day with his roughly 1,500 to 2,000 men despite many orders. When the battle was lost, he didn't break out sideways but straight through the advancing Eastern Army. Dozens of samurai stayed behind to hold off pursuers, a tactic called "sutegamari." Shimazu's nephew Toyohisa fell, Ii Naomasa was wounded in the arm by a musket ball during the pursuit, and Shimazu himself reached Satsuma with only about 80 men. Mitsunari fled into the Ibuki mountains, was captured three days after the battle, and soon afterward was executed with other Western leaders on the Kyoto riverbed. The number of dead is disputed: depending on the chronicle, between 8,000 and 35,000 men are said to have fallen on the Western side, and between 4,000 and 10,000 on the Eastern.

Ieyasu redistributed land and wealth, rewarding his generals' loyalty and punishing those who had opposed him. Mōri Terumoto, nominal commander in chief of the Western Army, who never took the field, saw his domain shrink from 1.2 million koku to 369,000, after Ieyasu found incriminating documents in Osaka Castle and voided the agreement with Kikkawa Hiroie. Kobayakawa Hideaki received 570,000 koku in Bizen and Mimasaka, the former holdings of the defeated Ukita, and died only two years later, on 1 December 1602, by most accounts aged 21. Ieyasu kept him at arm's length after the battle, and Hideaki was no longer popular. According to a legend, he was haunted by the ghost of Ōtani, whom he had betrayed.

On 24 March 1603, Emperor Go-Yōzei appointed Ieyasu shōgun, and the Tokugawa shogunate lasted until 1868. The child in whose name Mitsunari had fought survived Sekigahara by 15 years: Toyotomi Hideyori died in 1615 when the Tokugawa took Osaka Castle.

To wrap up, the short version, so fact and story don't blur.

If you've seen "Shōgun," you know the story as fiction: Lord Toranaga is inspired by Tokugawa Ieyasu, his rival Ishido by Ishida Mitsunari, and the English navigator Blackthorne goes back to William Adams, who met Ieyasu several times in May and June 1600. James Clavell's book ends after the Battle of Sekigahara.

Some of it is invention: unlike Ishido, Mitsunari was not a member of the Council of Five Elders but one of the five commissioners. And Mariko has a real model, Hosokawa Gracia, who died shortly before Sekigahara because she refused to be taken hostage by Mitsunari. She never met Blackthorne. Season 2 of the series, whose filming wrapped in September 2026, is set ten years later, and no premiere date has been set. The character Ochiba is based on Yodo-dono, the mother of Toyotomi Hideyori, about whom there's more in the Hideyoshi article.

Sekigahara shows why an era needs more than names and battles. That's why every season in HistoryBytes is split into five categories, and this story touches all five: Kobayakawa Hideaki as a person, Sekigahara as a place, the battle of 21 October 1600 as an event, the switching of sides as an idea, the matchlock musket, called teppō in Japanese, as an object. That's how the Whispers of the Samurai season builds the whole picture of a time, how people lived, fought, and decided, not just a list of battles.

Sekigahara lasted only a few hours and ended an era that had shaped Japan for over a century. It's a rare case in which a battle truly counts as a turning point, and at the same time an example of how turning points are often decided beforehand: by letters, by promises, by men who simply did nothing in the decisive hour. The image of the warning shot may be dramatic. What the sources better support is less spectacular and therefore perhaps more unsettling: the winner was settled before the fog lifted.