Battle of Sekigahara - Beginning

Beginning

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 Hasedō, 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.