Oda Nobunaga had slowly consolidated control over much of Japan and was in control of the shōgun, Ashikaga Yoshiaki. Ashikaga tried to escape this predicament in 1573 by attacking Oda, but failed and was exiled, thus ending his shōgunate. Oda ruled unopposed until he was betrayed by his own retainer Akechi Mitsuhide in 1582. While under attack in Kyoto, Oda committed suicide by seppuku (not proved). Toyotomi Hideyoshi quickly avenged his master Oda and consolidated control over Japan. Toyotomi had risen from humble roots – his father was an ashigaru (foot-soldier) – to become the ruler of Japan. His death created a power vacuum which ultimately was resolved by the outcome at Sekigahara.
Even though Toyotomi Hideyoshi unified Japan and consolidated his power following the Siege of Odawara in 1590, his failures in his invasions of Korea significantly weakened the Toyotomi clan's power as well as the support of the loyalists and bureaucrats who continued to serve and support the Toyotomi clan after Hideyoshi's death during the second invasion. The presence of Hideyoshi and his brother Hidenaga kept the two main factions of the time, which rallied behind Ishida Mitsunari and Tokugawa Ieyasu respectively, from anything more than quarrelling, but when both of them died, the conflicts were exacerbated and developed into open hostilities. With no appointed shōgun over the armies, this left a power vacuum in the Japanese government.
Most notably, Katō Kiyomasa and Fukushima Masanori were publicly critical of the bureaucrats, especially Ishida Mitsunari and Konishi Yukinaga. Tokugawa Ieyasu took advantage of this situation, and recruited them, redirecting the animosity to weaken the Toyotomi clan.