Warmatrix

The Battle of Sekigahara in 1600 CE was a turning point in Japanese history. It paved the way for the Tokugawa shogunate, which would lock the country down for 2½ centuries. The battle itself was decided as much by divided loyalties as by the strength of the armies involved.
The 1½ centuries before the battle are known as "Sengoku Jidai", the 'Warring States Period', when daimyo fought each other in an almost perpetual civil war. At the end of the 16th century CE, Oda Nobunaga nearly and then Toyotomi Hideyoshi fully unified Japan again. Yet neither of them had the ancestry to claim the title of shogun. When Hideyoshi died, the third powerful warlord, Tokugawa Ieyasu, quickly moved to take his place and installed himself in the capital, Edo. However other pretenders did not give in right away.
The greatest opposition to the Tokugawa came from Ishida Mitsunari, who formed an alliance against them. Most of these clans came from the west of Japan, so their army was called 'the army of the west'. The Uesugi clan provoked the Tokugawa to attack and this allowed Ishida to move to to their rear. But Ieyasu was not fooled. He let his allies deal with Uesugi and moved his main army west to meet the Ishida. In order to lure the westerners out, Ieyasu captured the important Gifu castle before his enemy could reach it. Mitsunari, nearby but drenched by rain, was afraid to be outflanked and took up a defensive position at Sekigahara. This paved the way for a field battle, exactly what Ieyasu wanted. His vanguard scouted ahead and soon ran into the ran into the Ishida position. Both sides briefly panicked, withdrew and lined up for battle. Mitsunari commanded a vast army of 80,000 men; Ieyasu had 74,000. Both sides included gunners armed with tanegashima, Japanese arquebuses.
The early morning was obscured by mist. When it cleared, the fighting erupted. Fukushima Masanori, on the Tokugawa left flank, opened the battle with a fierce attack, which largely got stuck in the mud. Ieasyu ordered an advance on the other fronts to support the attack. Fukushima's assault had left his flank vulnerable and Otani Yoshitsugu on the Ishida side used the opening to counterattack. Kobayakawa Hideaki, also on the side of the western army, had agreed to side with the Tokugawa, but now hesitated to move. Gunfire from Ieyasu's arquebusiers compelled him to act. He switched sides and charged Otani. However Otani too had arquebusiers, with dry gunpowder. They blasted away at the traitors.
Despite the failed attack, Otani was heavily outnumbered. Suddenly four other generals switched sides too. Otani was forced to retreat and the right flank of the western army crumbled; Otani himself committed suicide. Mitsunari wisely retreated, but was betrayed by another general, Kikkawa Hiroie. This caused the entire Ishida army to disintegrate. Some leaders escaped; others were captured; Mitsunari was taken after three days and executed. Both sides had reserves that arrived too late to take part in the battle.
At the time, the battle was not considered to be very important, but it cleared the way for Ieyasu's rise to power. He became shogun in 1603 CE. He proceeded to ban firearms, christians and almost all foreign trade, locking the country down. The final threats, Osaka castle and Hideyoshi's son Hideyori, were taken in 1614 CE.
The Tokguawa shogunate endured for 2½ centuries. When it finally cracked in the 18th century CE, dissident clans from Sekigahara were among the first to lead the Meji Restoration that toppled it.