About Battle Of Sekigahara 21 October 1600 Part.1

Initially, Ieyasu's eastern army had 75,000 men, while Mitsunari's western army numbered 120,000. Tokugawa had also sneaked in a supply of arquebuses. Knowing that Ieyasu was heading toward Osaka, Mitsunari decided to abandon his positions and marched to Sekigahara. Even though the Western forces had tremendous tactical advantages, Ieyasu had already been in contact with many daimyo in the Western Army for months, promising them land and leniency after the battle should they switch sides.

Tokugawa’s forces started the battle when Fukushima Masanori, the leader of the advanced guard, charged north from Tokugawa’s left flank along the Fuji River against the Western Army’s right center. The ground was still muddy from the previous day's rain, so the conflict there devolved into something more primal. Tokugawa then ordered attacks from his right and his center against the Western Army’s left in order to support Fukushima’s attack.

This left the Western Army’s center unscathed, so Ishida ordered this unit under the command of Shimazu Yoshihiro to reinforce his right flank. Shimazu refused as daimyo of the day only listened to respected commanders, which Ishida was not.

Fukushima’s attack was slowly gaining ground, but this came at the cost of exposing their flank to attack from across the Fuji River by Otani Yoshitsugu, who took advantage of this opportunity. Just past Otani’s forces were those of Kobayakawa Hideaki on Mount Matsuo.

Kobayakawa was one of the daimyo that had been courted by Tokugawa . Even though he had agreed to defect to Ieyasu's side, in the actual battle he was hesitant and remained neutral. As the battle grew more intense, Ieyasu finally ordered arquebuses to fire at Kobayakawa's position on Mount Matsuo in order to force Kobayakawa to make his choice. At that point Kobayakawa joined the battle as a member of the Eastern Army. His forces charged Otani's position, which did not end well for Kobayakawa. Otani’s forces had dry gunpowder, so they opened fire on the turncoats, making the charge of 16,000 men mostly ineffective. However, he was already engaging forces under the command of Tōdō Takatora, Kyogoku Takatsugu, and Oda Yuraku when Kobayakawa charged. At this point, the buffer Otani established was outnumbered. Seeing this, Western Army generals Wakisaka Yasuharu, Ogawa Suketada, Akaza Naoyasu, and Kutsuki Mototsuna switched sides, turning the tide of battle.
Source : Wikipedia