The Battle of Sekigahara (Shinjitai: ; Kyūjitai: , Hepburn romanization: Sekigahara no Tatakai) was a decisive battle on October 21, 1600 (Keichō 5, 15th day of the 9th month) in what is now Gifu prefecture, Japan, at the end of the Sengoku period. This battle was fought by the forces of Tokugawa Ieyasu against a coalition of Toyotomi loyalist clans under Ishida Mitsunari, several of which defected before or during the battle, leading to a Tokugawa victory. The Battle of Sekigahara was the largest battle of Japanese feudal history and is often regarded as the most important. Toyotomi's defeat led to the establishment of the Tokugawa shogunate.
Tokugawa Ieyasu took three more years to consolidate his position of power over the Toyotomi clan and the various daimyō, but the Battle of Sekigahara is widely considered to be the unofficial beginning of the Tokugawa shogunate, which ruled Japan for another two and a half centuries until 1868.
Background
Toyotomi clan rule
Toyotomi Hideyoshi was a prominent general under Oda Nobunaga. Nobunaga unified much of Japan under his rule after defeating the Shōgun Ashikaga Yoshiaki and ending the Ashikaga shogunate; however, he was betrayed by Akechi Mitsuhide and died at the Honnō-ji Incident of 1582. Hideyoshi quickly avenged his master and consolidated control over Japan afterward, with the aid of his brother Hidenaga. Hideyoshi had risen from humble roots—his father having been an ashigaru (foot soldier)—to become the ruler of Japan. To bolster his claim, Hideyoshi married noble women so that his heirs at least would descend from suitably distinguished families.
The final years of Hideyoshi's reign were troubled. While rivals in the Hojo clan were defeated at the Siege of Odawara in 1590, failures in the invasions of Korea significantly weakened the Toyotomi clan's power and its support from bureaucrats who served in the government. Additionally, Hideyoshi ordered the execution of Toyotomi Hidetsugu and his entire family in 1595. Hidetsugu was his nephew and heir, as well as the regent at the time who had been expected to take up leadership after Hideyoshi. When Hideyoshi was on his deathbed in 1598, he set up a regency government, as his new heir, Toyotomi Hideyori, was only five years old. Hideyoshi's death created a power vacuum; there was no appointed shōgun over the armies. The respected regent Maeda Toshiie, a neutral party between the clashing factions, kept the peace for a time, but he too died in 1599.
Feuding factions
Two main factions arose during the fading years of Hideyoshi's rule and the immediate aftermath of his death. Tokugawa Ieyasu was unrivaled in terms of seniority, rank, reputation and overall influence within the regency government, and had the allegiance of many of the lords of eastern Japan. Toyotomi clan loyalists and the lords of western Japan rallied behind Ishida Mitsunari. Tensions between them sometimes boiled into open hostilities, with relations eventually degenerating into the conflicts of 1600 that led to the Battle of Sekigahara.
Katō Kiyomasa and Fukushima Masanori were publicly critical of the bureaucrats, especially Mitsunari and Konishi Yukinaga. Tokugawa Ieyasu took advantage of this situation and recruited them, redirecting the animosity to weaken the Toyotomi clan. Rumours started to spread stating that Ieyasu, at that point the only surviving contemporary 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 Maeda Toshiie's son, Toshinaga, were accused of taking part and forced to submit to Ieyasu's authority. 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, Kagekatsu's chief advisor, Naoe Kanetsugu, responded with a counter-condemnation that mocked Ieyasu's abuses and violations of Hideyoshi's rules; Ieyasu was infuriated.
Afterwards, Ieyasu summoned the help of various supporters and led them northward to attack the Uesugi clan. However, many of them were at that moment besieging Hasedō. Ishida Mitsunari, grasping the opportunity created by the chaos, rose up in response and created an alliance to challenge Ieyasu's supporters.