After Toyotomi Hideyoshi's death in 1598, the land became split between Toyotomi supporters backing
Ishida Mitsunari and those that supported
Tokugawa Ieyasu, eventually culminating in the
Battle of Sekigahara in 1600. According to novels (
Rakusuishū 『落穂集』) of the Edo Period, Yoshihiro was initially supposed to Ieyasu's side. He was crushed by
Ishida Mitsunari's army upon his arrival for a rescue
Torii Mototada during the siege of Fushimi Castle and, after being humiliated, he took the side of Ishida Mitsunari instead. However, recent research indicated that Yoshihiro had chosen to side with Mitsunari from the very start, before
Mōri Terumoto announced the declaration of war against the Tokugawa in July 1600. Yoshihiro also played an important role as a member of Mitsunari's core staff and was pivotal in persuading
Uesugi Kagekatsu to ally with them. On September 13, Shimazu Yoshihiro led his soldiers to besiege Sone Castle and bombarding Sone castle with their artilleries.
Ii Naomasa and
Honda Tadakatsu implored
Mizuno Katsunari to repulse the Shimazu forces. In response, Katsunari went out with sallying force with his younger brother, Mizuno Tadatane, to defend Sone Castle. Katsunari ordered his artilleries to return fire at the turret of the Shimazu artilleries, then he led his army to storm the Shimazu position and manage to overcome the
Shimazu clan's army, causing Yoshihiro to retreat and abandon the siege against Sone castle. After he beat the Shimazu forces, returned to Ieyasu to ask permission to participate on the Sekigahara main battle which planned in the next day. However, Ieyasu instead ordered Katsunari to guard Sone castle and keep an eyes on Ogaki castle which located nearby and being controlled by Western army loyal to Mitsunari. As Katsunari burned the outer citadels of Ogaki castle, the Western army nearby that area, including the Shimazu clan who had just been beaten by Katsunari's army, decided to abort their plan to go in as they thought the Ogaki castle could not be saved. Then Yoshihiro decided to retreat into
Ise Province. According to the scripts of Yoshihiro's subordinate
Kando Kutarō, Yoshihiro got along with Mitsunari from the start, but their relationship was distorted by the novelists of the Edo Period. It was said that Mitsunari did not listen to any of Yoshihiro's plans, including a surprise night attack on the day before the actual battle of Sekigahara. On the day of the battle, Yoshihiro and his 1500 men held ground and did not fight at all. After the rest of Mitsunari's side was wiped out, Yoshihiro was stranded among at least 30,000 of Ieyasu's troops. Vastly outnumbered, Yoshihiro tried to make a charge against Ieyasu himself but after his nephew
Shimazu Toyohisa demanded that he not kill himself over a meaningless battle, Yoshihiro instead chose to charge straight through Ieyasu's troop to make an exit to the other side. Yoshihiro had his troops make a fighting retreat called
Sutegamari (捨て奸), where until a certain number of men died holding a position and repelling an attack, the main body of the army fought as well. Though Toyohisa and the bulk of the troops died, the charge and retreat were a success with a wound dealt to
Ii Naomasa in the process. After beating back the chase, he picked up his wife at Sumiyoshi in
Settsu Province and returned to
Satsuma Province by ships. The Japanese historian
Shiramine Jun studied why the Shimazu clan behaved torpidly and deduced it was because Yoshihiro had become involved in the power struggling between his brother Yoshihisa and
Ijūin Tadamune. It led Yoshihiro losing the support of Yoshihisa, which trapped Yoshihiro due to the lack of support from the Shimazu clan. == Death ==