Early life Mogami Yoshiaki was born on the first day of the first month of the Tenbun era (1546) to his father, Yoshimori,
Unification of Dewa province In 1571, Mogami Yoshiaki officially became the head of the Mogami clan. Yoshiaki married a woman from the Osaki clan of the Mutsu province, who was his relative. When Date later attacked the Osaki clan the relationship between Yoshimitsu and Date Masamune deteriorated. In 1586, Yoshiaki fought against Onodera Yoshimichi. Yoshiaki's eldest son, Mogami Yoshiyasu and
Tateoka Mitsushige succeeded in defeating the Onodera reinforcements. In October, Yoshiaki crossed the Rokuji Highway and invaded
Shōnai in an effort to reinforce Tōzenji Yoshinaga, lord of Tozenji Castle, against the attack of his enemy, Daihōji Yoshioki. Yoshioki was defeated in battle the same month and committed suicide, causing the Daihōji clan to go extinct. In 1587, he succeeded in taking control of Shonai (western Yamagata prefecture). However, the next year, his army was defeated and routed by
Honjō Shigenaga, a retainer of
Uesugi Kagekatsu, in
Battle of Jugorihara. This defeat, along with the fact that his nephew,
Date Masamune, was also expanding his own ambitions at the same time, and he was targeting domains within the Mogami clan, prompted Yoshiaki to give up Shōnai and move his focus towards the
Date clan.
Incident of Yoshiaki daughter In 1591, after the
Siege of Odawara, he participated in the
Kunohe Rebellion. During this time,
Toyotomi Hidetsugu, who had been chosen as Toyotomi Hideyoshi's successor, fell in love with Yoshiaki's daughter, Komahime, and married her, and Yoshiaki became an important Toyotomi daimyo as because of his relation to the
Toyotomi clan. In 1591, Yoshiaki was allowed to use the surname of Toyotomi, and usually called himself Hashiba Yoshiaki. Although he did not end up sailing to Korea during the Bunroku Invasion, in 1592, he went to Nagoya Castle in Hizen with 500 of his retainers to prepare for passage to Korea.
Sekigahara Campaign In 1600, when the
Sekigahara campaign broke out between Ieyasu and his enemies led by
Ishida Mitsunari, Ieyasu entrusted Yoshiaki with command of northern Oshu. On April 19, Ieyasu instructed the
Nanbu, Onodera, and Akita clans to obey Yoshiaki's command, The Uesugi forces which numbered more than 20,000, were invaded from the south by the main force commanded by Naoe Kanetsugu via the Kitsunegoe Highway, and from the west by the Shonai area. Because the difference in military strength between the Mogami forces and the Uesugi forces were huge, Yoshiaki requested reinforcements from Date Masamune. The Uesugi forces were defeated in the attack on Kaminoyama Castle, and fled back to Yonezawa in disarray. In the 15 days of ineffective siege against the Hasedō Castle, the Uesugi troops cannot capture it from the persistent Mogami forces. In the final phase of this siege, the Uesugi forces launched another all-out attack on Hasedō Castle, but with the help of Date's reinforcements, they were able to hold out against the large Uesugi forces. In the end, when information came in that Tokugawa Ieyasu emerged victorious in the main
Sekigahara battle, The uesugi forces under Kanetsugu began to withdraw from Hasedō Castle. Switching positions, Yoshiaki began to pursue them, but Kanetsugu himself concentrated his riflemen to defend his retreating army, and managed to withdraw safely back to Yonezawa. After the campaign ended, the Mogami clan was rewarded with an increase in size from 200,000 to 570,000
koku for their domain in return for their loyalty to the Eastern army during the war. Yamagata maintains the
Mogami Yoshiaki Historical Museum, just outside the rebuilt Great Eastern Gate of Yamagata Castle, which displays his helmet, battle command baton and other implements he actually used. It was reported that during his reign as Mogami clan's head not a single local uprising occurred within the territories of Mogami clan. ==Legacy==