|Commanders of Eastern Army (Tokugawa Force)|
|Tokugawa Ieyasu - 30,000 men|
|Kato Kiyomasa - 3,000 men|
|Fukushima Masanori - 6,000 men|
|Hosokawa Tadaoki - 5,000 men|
|Asano Yukinaga - 6,510 men|
|Ikeda Terumasa - 4,560 men|
|Kuroda Nagamasa - 5,400 men|
|Kato Yoshiaki - 3,000 men|
|Tanaka Yoshimasa - 3,000 men|
|Todo Takatora - 2,490 men|
|Yamauchi Katsutoyo - 2,058 men|
|Hachisuka Yoshishige -|
|Honda Tadakatsu - 500 men|
|Terasawa Hirotaka - 2,400 men|
|Ikoma Kazumasa - 1,830 men|
|Ii Naomasa - 3,600 men|
|Matsudaira Tadayoshi - 3,000 men|
|Oda Nagamasu - 450 men|
|Tsutsui Sadatsugu - 2,850 men|
|Kanamori Nagachika - 1,140 men|
|Furuta Shigekatsu - 1,200 men|
|Arima Toyouji - 900 men|
|Commanders of Western Army (Ishida Force)|
|Mori Terumoto (official head of the alliance) - (not present)|
|Maeda Toshimasa (Brother of Maeda Toshinaga)|
|Ukita Hideie - 17,000 men|
|Shimazu Yoshihiro - 1,500 men|
|Kobayakawa Hideaki (defected) - 15,600 men|
|Ishida Mitsunari (de facto head of the alliance) - 4,000 men|
|Konishi Yukinaga - 4,000 men|
|Ogawa Suketada (defected) - 2,100 men|
|Otani Yoshitsugu - 600 men|
|Wakisaka Yasuharu (defected) - 990 men|
|Ankokuji Ekei - 1,800 men|
|Chosokabe Morichika - 6,600 men|
|Kutsuki Mototsuna (defected) - 600 men|
|Akaza Naoyasu (defected) - 600 men|
|Kikkawa Hiroie (defected) - 3,000 men|
|Natsuka Masaie - 1,500 men|
|Mori Hidemoto - 15,000 men|
|Toda Katsushige - 1,500 men|
|Shima Sakon - 1,000 men|
The , popularly known as the , was a decisive battle on October 21, 1600 (Keichō 5, 15th day of the 9th month) which cleared the path to the Shogunate for Tokugawa Ieyasu. Though it would take three more years for Ieyasu to consolidate his position of power over the Toyotomi clan and the daimyo, Sekigahara is widely considered to be the unofficial beginning of the Tokugawa bakufu, the last shogunate to control Japan.
Later, a supposed conspiracy to assassinate Ieyasu surfaced, and many Toyotomi loyalists, including Toshiie's son, Toshinaga, were accused of taking part and forced to submit to Ieyasu's authority. However, Uesugi Kagekatsu, one of Hideyoshi's appointed regents, defied Ieyasu by building up his military. When Ieyasu officially condemned him and demanded that he come to Kyoto to explain himself before the emperor, Kagekatsu's chief advisor, Naoe Kanetsugu responded with a counter-condemnation that mocked Ieyasu's abuses and violations of Hideyoshi's rules, in such a way that Ieyasu was infuriated.
Afterwards, Ieyasu summoned the help of various supporters and led them northward to attack the Uesugi clan, which at that moment were besieging Hasedō, but Ishida Mitsunari, grasping the opportunity, rose up in response and created an alliance to challenge Ieyasu's supporters, also seizing various daimyo as hostages in Osaka Castle. Ieyasu then left some forces led by Date Masamune to keep the Uesugi in check and marched west to confront the western forces. A few daimyo, most notably Sanada Masayuki, left Ieyasu's alliance, although most, either bearing grudges against Mitsunari or being loyal to Ieyasu, stayed with him.
Mitsunari then officially declared war on Ieyasu and lay siege to the Fushimi Castle, garrisoned by Tokugawa retainer Torii Mototada on July 19. Afterwards, the western forces captured various Tokugawa outposts in the Kansai region and within a month, the western forces had moved into the Mino Province, where Sekigahara was located.
Back in Edo, Ieyasu received news of the situation in Kansai and decided to deploy his forces. He had some former Toyotomi daimyo engage with the western forces while he split his troops and marched west on the Tōkaidō towards Osaka Castle. Ieyasu's son Hidetada led another group through Nakasendō. However, Hidetada's forces were bogged down as he attempted to besiege Sanada Masayuki's Ueda Castle. Even though the Tokugawa forces numbered some 38,000, an overwhelming advantage over the Sanada's mere 2,000, they were still unable to capture the strategist's well-defended position. At the same time, 15,000 Toyotomi troops were being held up by 500 troops under Hosokawa Fujitaka at Tanabe Castle in Wakayama Prefecture. Some among the 15,000 troops respected Hosokawa so much they intentionally slowed their pace down. Both these incidents resulted in a large number of Tokugawa and Toyotomi troops not to show up in time at the battlefield of Sekigahara.
Knowing that Ieyasu was heading toward Osaka, Mitsunari decided to abandon his positions and marched to Sekigahara. On September 15, 1600 (Keichō 5, 8th day of the 8th month), the two sides started to deploy their forces. Ieyasu's eastern army had 88,888 men, whilst Mitsunari's western army numbered 81,890. There were about 20,000 arquebusers and other forms of hand-held gunners deployed in the battlefield, corresponding to over 10% of all troops present.
Even though the western forces had tremendous tactical advantages, Ieyasu had already contacted many daimyo on the western side, promising them land and leniency after the battle should they switch sides. This led some western commanders holding key positions to hesitate when pressed to send in reinforcements or join the battle that was already in progress.
Mori Hidemoto and Kobayakawa Hideaki were two such daimyo. They were in such positions that if they decided to close in on the eastern forces, they would in fact have Ieyasu surrounded on three sides. Hidemoto, shaken by Ieyasu's promises, also persuaded Kikkawa Hiroie not to take part in the battle.
Even though Kobayakawa had responded to Ieyasu's call, he remained hesitant and neutral. As the battle grew more intense, Ieyasu finally ordered arquebusiers to fire at Kobayakawa's position on Mount Matsuo. At that point Kobayakawa joined the battle on the eastern side. His forces assaulted Yoshitsugu's position, which quickly fell apart as he was already engaging Todo Takatora's forces. Seeing this as an act of treachery, western generals such as Wakisaka Yasuharu, Ogawa Suketada, Akaza Naoyasu, and Kutsuki Mototsuna immediately switched sides, turning the tide of battle.
The western forces disintegrated afterwards, and the commanders scattered and fled. Some, like Ukita Hideie managed to escape, while others, like Sakon was shot and wounded by a rifle though it's unknown if he died from it, Otani Yoshitsugu committed suicide. Mitsunari, Yukinaga and Ekei were some of those who were captured and a few, like Mori Terumto and Shimazu Yoshihiro were able to return to their home provinces. Mitsunari himself would be decapitated.
Tokugawa Ieyasu redistributed the lands and fiefs of the participants, generally rewarding those who assisted him and displacing, punishing, or exiling those who fought against him. In doing so, he gained control of many former Toyotomi territories. Following the public execution of Ishida Mitsunari, Konishi Yukinaga and Ankokuji Ekei, the influence and reputation of the Toyotomi clan and its remaining loyalists drastically decreased.
From the Toyotomi clan's point of view though, the battle was technically only an internal conflict between Toyotomi vassals.
This view was challenged however because Ieyasu was later made Shogun, a position that had been left vacant since the fall of the Ashikaga shogunate 27 years earlier. In 1664, Tokugawa historian and Yushima Seido rector Hayashi Gahō, writing in 1664, summarized the consequences of the battle: "Evil-doers and bandits were vanquished and the entire realm submitted to Lord Ieyasu, praising the establishment of peace and extolling his martial virtue. That this glorious era that he founded may continue for ten thousands upon ten thousands of generations, coeval with heaven and earth.
This change in official rankings also reversed the subordinate position of the Tokugawa clan, thus making the Toyotomi clan subordinates of the Tokugawa instead. In any case, Ieyasu did not gain any casus belli to take action against the frail Toyotomi clan; rather, it would take more political maneuvers for Ieyasu to destroy Hideyori once and for all.
The descendants of these three clans would in two centuries collaborate to bring down the Tokugawa shogunate, leading to the Meiji Restoration.