The Battle of Sekigahara (Shinjitai: Kyūjitai: Sekigahara no Tatakai?), popularly known as the Realm Divide (Tenka Wakeme no Tatakai), was a decisive battle on October 21, 1600 (Keichō 5, 15th day of the 9th month) which cleared the path to the Shogunate for Tokugawa Ieyasu. Though it would take three more years for Ieyasu to consolidate his position of power over the Toyotomi clan and the daimyo, Sekigahara is widely considered to be the unofficial beginning of the Tokugawa bakufu, the last shogunate to control Japan.
Even though Toyotomi Hideyoshi unified Japan and consolidated his power following the Siege of Odawara in 1590, his Japanese invasions of Korea (1592-1598) significantly weakened the Toyotomi clan's power as well as the loyalists and bureaucrats that continued to serve and support the Toyotomi clan after Hideyoshi's death. Hideyoshi's and his brother Hidenaga's presence kept the two sides from anything more than quarreling, but when both of them died, the conflicts were exacerbated and developed into open hostilities. Since the Toyotomi clan was known to be descended from peasant stock, neither Hideyoshi nor his heir Hideyori would be recognized or accepted as Shogun.
Most notably, Katō Kiyomasa and Fukushima Masanori were publicly critical of the bureaucrats, especially Ishida Mitsunari and Konishi Yukinaga. Tokugawa Ieyasu took advantage of this situation, and recruited them, redirecting the animosity to weaken the Toyotomi clan.
Tokugawa Ieyasu was no longer rivaled in terms of seniority, rank, reputation and overall influence within the Toyotomi clan after the death of Regent Maeda Toshiie. Rumors started to spread stating that Ieyasu, at that point the only surviving ally of Oda Nobunaga, would take over Hideyoshi's legacy just as Nobunaga's was taken. This was especially evident amongst the loyalist bureaucrats, who suspected Ieyasu of agitating unrest amongst Toyotomi's former vassals.
Later, a supposed conspiracy to assassinate Ieyasu surfaced, and many Toyotomi loyalists, including Toshiie's son, Toshinaga, were accused of taking part and forced to submit to Ieyasu's authority. However, Uesugi Kagekatsu, one of Hideyoshi's appointed regents, defied Ieyasu by building up his military. When Ieyasu officially condemned him and demanded that he come to Kyoto to explain himself before the emperor, Kagekatsu's chief advisor, Naoe Kanetsugu responded with a counter-condemnation that mocked Ieyasu's abuses and violations of Hideyoshi's rules, in such a way that Ieyasu was infuriated.
Afterwards, Ieyasu summoned the help of various supporters and led them northward to attack the Uesugi clan, which at that moment were besieging Hasedō, but Ishida Mitsunari, grasping the opportunity, rose up in response and created an alliance to challenge Ieyasu's supporters, also seizing various daimyo as hostages in Osaka Castle.
Ieyasu then left some forces led by Date Masamune to keep the Uesugi in check and marched west to confront the western forces. A few daimyo, most notably Sanada Masayuki, left Ieyasu's alliance, although most, either bearing grudges against Mitsunari or being loyal to Ieyasu, stayed with him
Mitsunari, in his home Sawayama Castle, met with Ōtani Yoshitsugu, Mashita Nagamori, and Ankokuji Ekei. Here, they forged the alliance, and invited Mōri Terumoto, who actually did not take part in the battle, to be its head.
Mitsunari then officially declared war on Ieyasu and lay siege to the Fushimi Castle, garrisoned by Tokugawa retainer Torii Mototada on July 19. Afterwards, the western forces captured various Tokugawa outposts in the Kansai region and within a month, the western forces had moved into the Mino Province, where Sekigahara was located.
Back in Edo, Ieyasu received news of the situation in Kansai and decided to deploy his forces. He had some former Toyotomi daimyo engage with the western forces while he split his troops and marched west on the Tōkaidō towards Osaka Castle.
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. 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 Fujitaka at Tanabe Castle in Wakayama Prefecture. Some among the 15,000 troops respected Hosokawa so much they intentionally slowed their pace down. Both these incidents resulted in a large number of Tokugawa and Toyotomi troops not to show up in time at the battlefield of Sekigahara.
Knowing that Ieyasu was heading toward Osaka, Mitsunari decided to abandon his positions and marched to Sekigahara. On September 15, 1600 (Keichō 5, 8th day of the 8th month), the two sides started to deploy their forces. Ieyasu's eastern army had 88,888 men, whilst Mitsunari's western army numbered 81,890. There were about 20,000 arquebusers and other forms of hand-held gunners deployed in the battlefield, corresponding to over 10% of all troops present.
Even though the western forces had tremendous tactical advantages, Ieyasu had already contacted many daimyo on the western side, promising them land and leniency after the battle should they switch sides. This led some western commanders holding key positions to hesitate when pressed to send in reinforcements or join the battle that was already in progress.
Mōri Hidemoto and Kobayakawa Hideaki were two such daimyo. They were in such positions that if they decided to close in on the eastern forces, they would in fact have Ieyasu surrounded on three sides. Hidemoto, shaken by Ieyasu's promises, also persuaded Kikkawa Hiroie not to take part in the battle.
Even though Kobayakawa had responded to Ieyasu's call, he remained hesitant and neutral. As the battle grew more intense, Ieyasu finally ordered arquebusiers 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 on the eastern side. His forces assaulted Yoshitsugu's position, which quickly fell apart as he was already engaging Tōdō Takatora's forces. Seeing this as an act of treachery, western generals such as Wakisaka Yasuharu, Ogawa Suketada, Akaza Naoyasu, and Kutsuki Mototsuna immediately switched sides, turning the tide of battle.
The western forces disintegrated afterwards, and the commanders scattered and fled. Some, like Ukita Hideie managed to escape, while others, like Sakon was shot and wounded by a rifle though it's unknown if he died from it, 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.
Tokugawa Ieyasu redistributed the lands and fiefs of the participants, generally rewarding those who assisted him and displacing, punishing, or exiling those who fought against him. In doing so, he gained control of many former Toyotomi territories. Following the public execution of Ishida Mitsunari, Konishi Yukinaga and Ankokuji Ekei, the influence and reputation of the Toyotomi clan and its remaining loyalists drastically decreased.
At the time, the battle was considered only an internal conflict between Toyotomi vassals. However, after Ieyasu later became Shogun, a position that had been left vacant since the fall of the Ashikaga shogunate 27 years earlier, the battle was perceived as more important event. In 1664, Hayashi Gahō, Tokugawa historian and rector of Yushima Seido, summarized the consequences of the battle: "Evil-doers and bandits were vanquished and the entire realm submitted to Lord Ieyasu, praising the establishment of peace and extolling his martial virtue. That this glorious era that he founded may continue for ten thousands upon ten thousands of generations, coeval with heaven and earth."
This change in official rankings also reversed the subordinate position of the Tokugawa clan, thus making the Toyotomi clan subordinates of the Tokugawa instead.
While most clans were content with their new status, there were many clans, especially those on the western side, who became bitter about their displacement or what they saw as a dishonorable defeat or punishment. Three clans in particular did not take the aftermath of Sekigahara lightly:
The Mōri clan, headed by Mōri Terumoto, remained angry toward the Tokugawa shogunate for being displaced from their fief, Aki, and being relocated to the Chōshū Domain, even though the clan did not take part in the battle at all.
The Shimazu clan, headed by Shimazu Yoshihiro, blamed the defeat on its poor intelligence-gathering, and while they were not displaced from their home province of Satsuma, they did not become completely loyal to the Tokugawa shogunate either. Taking advantage of its large distance between Edo and the island of Kyūshū as well as its improved espionage, the Shimazu clan demonstrated that it was virtually an autonomous kingdom independent from the Tokugawa shogunate during its last days.
The Chōsokabe clan, headed by Chōsokabe Morichika, was stripped of its title and domain of Tosa and sent into exile. Former Chōsokabe retainers never quite came to terms with the new ruling family, the Yamauchi clan, which made a distinction between its own retainers and former Chōsokabe retainers, giving them lesser status as well as discriminating treatment. This class distinction continued even generations after the fall of the Chōsokabe clan.
The descendants of these three clans would in two centuries collaborate to bring down the Tokugawa shogunate, leading to the Meiji Restoration.
According to tradition, the legendary kensei Miyamoto Musashi was present at the battle among the ranks of Ukita Hideie's army. Supposedly, he fought well and escaped the defeat of Hideie's forces unharmed. Whether this is fact or myth is unknown, considering that Musashi would have been around 17 years of age at the time. Several books on Japanese history, martial arts and the biography proceeding a translation of the 'Go Rin no Sho' (see source) mention Musashi joining the ranks of the Ashikaga army and escaping the hunting down and massacre of the vanquished army. More curious is the fact that he joined Tokugawa in both winter and summer campaigns of 1614 and 1615 when Ieyasu laid siege to Osaka castle where supporters of the Ashikaga family gathered in insurrection, thereby fighting against those he had originally fought for at Sekigahara. According to legend, he had already killed in self defense at 13 and again at 16 defeating Tadashima; he then left home on his 'Warrior pilgrimage' which saw him victorious in scores of contest and which took him to war before he was 17.