Background to the Battle of Sekigahara

Hideyoshi's Death

Hideyoshi Toyotomi succeeded to unify the whole country. He passed away in the midst of Imjin War, in Keicho 3 year. 5 chief ministers (Ieyasu Tokugawa, Toshiie Maeda, Hideie Ukita, Terumoto Mori, Takakage Kobayakawa) and 5 magistrates (Nagamasa Asano, Genni Maeda, Mitsunari Ishida, Masaie Natsuka, Nagamori Mashita) were left. Hideyoshi aimed at leaving peace and security of Toyotomi family.
However, since his successor Hideyori was only six years, Ieyasu Tokugawa defeated the rules of adaption between feudal lords. He gradually tried to expand his influence.

Attack of Mitsunari and Movement of Ieyasu

Toshiie Maeda died of sickness in March, in Keicho 4 years (1599). On the same day, the attack of Mitsunari was planned by Kiyomasa Kato and other 7 Toyotomi group's commanders who dissatisfied with the response of sending troops to Korea. However, Ieyasu changed their mind, and sent Mitsunari to Sawayama castle. Mitsunari was penitent there.
Ieyasu disposed of Nagamasa Asano and some other commanders on suspicion of planning to assassinate Mitsunari. He also made Family Maeda put under his control after Toshiie Maeda's death. Even after Ieyasu ignored Hideyasu's last will. He gave selfish increase in salary and relocation again and again.
Kagekatsu Uesugi was proceeding to the capital after the death of Hideyoshi. He returned to home in August Keicho 4 years (1599) to repair roads and bridges in his country. And he restored castles within the territory. His movement was obviously to strengthen the military forces.
Therefore, Ieyasu judged that Kagegatsu Uesugi was threatening and he announced the Aizu subjugation instead of magistrates objection. Ieyasu left Osaka in 16 June Keicho 5 years (1600). He took the field from Edo castle to Aizu in July 21.

Raising Army of Mitsunari

When Mitsunari saw Ieyasu's Aizu subjugation, he decided to overthrow Ieyasu to promote peace and security of the Toyotomi family. He didn't listen to Yoshitsugu Otani's persuasion. Mitsunari also seek the cooperation to Ekei in Ankokuji temple, Nagamori Mashita and Terumoto Mori. Terumoto became a chief commander of West forces at July 17 Keicho 5 years (1600) and he was welcomed to Osaka castle. They gave letters of impeachment charges of Ieyasu Tokugawa, the joint signature of three magistrates on it, to feudal lords. It was a declaration of war against Ieyasu Tokugawa. As a response of the letters, the feudal lords gathered to Osaka.
Mitsunari forces started to attack a Fushimi castle from 19 July. Fushimi castle fell on 1 August.
They followed Ieyasu from three streets, Tokai, Nakasen and Hokuriku. They wanted to attack Ieyasu from both sides with Kagekatsu Uesugi. Yoshitsugu Otani and Yasuharu Wakisaka went to Hokuriku. Mitsunari forces took Tosando and entered Ogaki castle on August 1.

Evaluation of Oyama

On the other hand, when Ieyasu heard of raising army of Mitsunari, for the Aizu subjugation, he had a conference of Oyama forces in Shimotsuke. In the conference, Masanori Fukushima who had been high in Toyotomi's favor, told to support Ieyasu. After that, other feudal lords who had been high in Toyotomi's favor, agreed to Masanori Fukushima. They decied to overthrow Mitsunari. Masanori Fukushima and Terumasa Ikeda joined with Naomasa Ii and Tadakatsu Honda on their way. They gathered to Kiyosu castle on August 14. He never move before he saw through the feudal lords who had been high in Toyotomi's favor, in Kiyosu castle.

Fall of Gifu Castle

Masanori Fukushima and Terumasa Ikeda were waiting for Ieyasu in Kiyosu castle. They had a military conference there on August 20. They divided by both advancing to attack Gifu castle. On August 22 they started to attack Gifu castle. Hidenobu Oda, the master of Gifu castle surrendered just one day. Reinforcements of Mitsunari turned back to Oogaki castle without fighting at all. Ieyasu heard of the news of victory. He left Edo on September 1 and arrived at Minoakasaka where Masanori Fukushima and Terumasa Ikeda were waiting for him at noon on 14, the day before the battle.

Western Army went to Sekigahara

Yoshitsugu Otani and Yasuharu Wakisaka took up their position from Hokuriku on September 3 in Yamanaka village in Sekigahara. Hidemoto Mori from Iseji took up his position on September 7 on Nangusan and Hideie Ukita entered Ogaki castle.
On September 14, Mitsunari knew Ieyasu's arrival at Akasakaokayama. Mitsunari provoked East forces to encourage West forces on Kuise river and brilliantly won the bettle (bettle of Kuise river).
Some people said to attack Ieyasu in the night. But Mitsunari got an information that East forces went to Osaka castle. In haste West forces made a detour to avoid south of Nangusan and went to Sekigahara. They took up their position on the west side of the Sekigahara Basin.

Eastern Army went to Sekigahara

Ieyasu received the report from Ogaki castle that main power of West forces headed towards Sekigahara. He gave an order to follow the Western army. At the daybreak of 15th the Eastern army took up their position on the east side of the Sekigahara Basin. They faced to the Western army.