The Battle of Sekigahara, popularly known as the Battle for the Sundered Realm, 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. Japan then had a long period of peace after that battle.
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 loyalists and bureaucrats that continued to serve and support the Toyotomi clan after Hideyoshi’s death. Hideyoshi’s and his brother Hidenaga’s presence kept the two sides from anything more than quarreling, but when both of them died, the conflicts were exacerbated and developed into open hostilities. Since the Toyotomi clan was known to be descended from peasant stock, neither Hideyoshi nor his heir Hideyori would be recognized or accepted as Shogun.
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.
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.
At the time, the battle was considered only an internal conflict between Toyotomi vassals. However, after Ieyasu became Shogun, a position that had been left vacant since the fall of the Ashikaga shogunate 27 years earlier, the battle was perceived as a more important event. In 1664, Hayashi Gahō, Tokugawa historian and rector of Yushima Seido, 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.
Source: Wikipedia, read more here