Tokugawa Ieyasu was no longer rivaled in terms of seniority, rank, reputation and overall influence within the Toyotomi clan after the death of Regent Maeda Toshiie. Rumors started to spread stating that Ieyasu, at that point the only surviving ally of Oda Nobunaga, would take over Hideyoshi's legacy just as Nobunaga's was taken. This was especially evident amongst the loyalist bureaucrats, who suspected Ieyasu of agitating unrest amongst Toyotomi's former vassals.
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 Hasedo, 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, in his home Sawayama Castle, met with Otani Yoshitsugu, Mashita Nagamori, and Ankokuji Ekei. Here, they forged the alliance, and invited Mouri Terumoto, who actually did not take part in the battle, to be its head.
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 Tokaidou towards Osaka Castle.
Ieyasu's son Hidetada led another group through Nakasendou. 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, 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.
Mouri 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 in order to force Kobayakawa to make his choice. 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 Toudou 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 Mouri Terumoto and Shimazu Yoshihiro were able to return to their home provinces. Mitsunari himself would be executed.