About Battle Of Sekigahara 21 October 1600 Part.2

Heavily outnumbered, Otani had no choice but to retreat. This left the Western Army’s right flank wide open, so Fukushima and Kobayakawa began to roll up it. Thus Ishida’s right flank was destroyed and his center was being pushed back, so he decided to retreat.

Ishida’s only remaining forces were on Mount Nangu. However, these forces were there for a reason. Kikkawa Hiroie was one of the commanders on the mountain. Kikkawa's troops formed the front lines of the Mori army, which was commanded by his cousin Mori Hidemoto. Earlier, when Hidemoto decided to attack the Tokugawa forces, Hiroie refused to comply, stating he was busy eating and asked to be left alone. This in turn prevented the Chosokabe army, which deployed behind the Mori clan, from attacking. When Ishida arrived, Kikkawa betrayed him as well. He kept the Mori army at bay, and since Ishida had no more support, he was defeated.

The Western Army disintegrated afterwards, and the commanders scattered and fled. Some, like Ukita Hideie managed to escape, at least initially. Many others did not. Shima Sakon was shot and fatally wounded by a round from an arquebus and Ōtani Yoshitsugu committed suicide. Mitsunari, Yukinaga and Ekei were some of those who were captured and a few, like Mōri Terumoto and Shimazu Yoshihiro were able to return to their home provinces. Mitsunari himself would be executed.

Both sides had forces that did not arrive at Sekigahara in time to participate due to other battles.

Ieyasu's son Hidetada led another group through Nakasendō. However, Hidetada's forces were bogged down as he attempted to besiege Sanada Masayuki's Ueda Castle against his father's direct orders. Even though the Tokugawa forces numbered some 38,000, an overwhelming advantage over the Sanada's mere 2,000, they were still unable to capture the strategist's well-defended position.

At the same time, 15,000 Toyotomi troops were being held up by 500 troops under Hosokawa Yusai at Tanabe Castle in present day Maizuru, Kyoto Prefecture. Some among the 15,000 troops respected Hosokawa so much they intentionally slowed their pace. Due to these incidents, a large number of Tokugawa- and Toyotomi-aligned troops failed to show up in time for the battle. Had either of these armies participated in the conflict, it could have ended quite differently.
Source: Wikipedia

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