Mitsunari, knowing that Ieyasu is heading toward Osaka, decides to abandon his positions and march to Sekigahara.

On September 15, 1600, the two sides start to deploy their forces.

Ieyasu's eastern army has eighty eight thousand eight hundred and eighty-eight men, whilst Mitsunari's western army numbers eighty-one thousand eight hundred and ninety.

There are about twenty thousand arquebusiers and other forms of hand-held gunners deployed in the battlefield, corresponding to over ten percent of all troops present.

The western forces have tremendous tactical advantages, but Ieyasu has already contacted many daimyo on the western side, promising them land and leniency after the battle should they switch sides.

This leads some western commanders holding key positions to hesitate when pressed to send in reinforcements or join the battle that is already in progress.

Two such daimyo, Mōri Hidemoto and Kobayakawa Hideaki, are in such positions that had 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 persuades Kikkawa Hiroie not to take part in the battle.

Kobayakawa, although he had responded to Ieyasu's call, remains hesitant and neutral.

As the battle grows more intense, Ieyasu finally orders arquebusiers to fire at Kobayakawa's position on Mount Matsuo in order to force Kobayakawa to make his choice.

At this point Kobayakawa joins the battle on the eastern side.

His forces assault Yoshitsugu's position, which quickly falls apart as he is already engaging Tōdō Takatora's forces.

Seeing this as an act of treachery, western generals such as Wakisaka Yasuharu, Ogawa Suketada, Akaza Naoyasu, and Kutsuki Mototsuna immediately switch sides, turning the tide of battle.

Tokugawa Ieyasu, proving himself an excellent strategist, is the victor of Japan’s great Battle of Sekigahara on October 21, 1600, in which he and his allies defeat Ishida Mitsunari and his allies, setting the stage for the rule of the Tokugawa shogunate.

This signals the end of the Azuchi-Momoyama period and the beginning of the Edo period.

Mitsunari is captured and executed as a war criminal and all but Hideyori and a few of his followers accept the new order.

Most clans are content with their new status, but there are many clans, especially those on the western side, who become bitter about their displacement or what they see as a dishonorable defeat or punishment.

Three clans in particular do not take lightly the aftermath of Sekigahara:

• The Mōri clan, headed by Mōri Terumoto, are to remain angry toward the Tokugawa shogunate for being displaced from their fief, Aki, and being relocated to the Chōshū Domain, even though the clan had not taken part in the battle at all.

• The Shimazu clan, headed by Shimazu Yoshihiro, blames the defeat on its poor intelligence-gathering, and while they are not displaced from their home province of Satsuma, neither do they they become completely loyal to the Tokugawa shogunate.

Taking advantage of its large distance between Edo and the island of Kyūshū as well as its improved espionage, the Shimazu clan is to demonstrate that it is virtually an autonomous kingdom independent from the Tokugawa shogunate during its last days.

• The Chōsokabe clan, headed by Chōsokabe Morichika, is stripped of its title and domain of Tosa and sent into exile.

Former Chōsokabe retainers will never quite came to terms with the new ruling family, the Yamauchi clan, which will make a distinction between its own retainers and former Chōsokabe retainers, giving them lesser status as well as discriminating treatment.

This class distinction is to continue even generations after the fall of the Chōsokabe clan.

The descendants of these three clans will in two centuries collaborate to bring down the Tokugawa shogunate, leading to the Meiji Restoration.