Why the Battle of Sekigahara was a pivotal moment in Japanese history

During the late 15th century, the collapse of the Ashikaga shogunate left Japan divided and without a central leader.

In the absence of central authority, leading daimyo governed their territories independently and relied on military strength to maintain control.

Continuous conflict disrupted stability for more than a hundred years. 

Under Oda Nobunaga, a wave of campaigns brought much of central Japan under his control.

After his death in 1582, Toyotomi Hideyoshi completed the work of unification. As the most powerful man in the country, he ruled without the title of shogun and governed as the emperor’s deputy. 

In 1593, the birth of his son, Hideyori, changed the balance of power. Hideyoshi anticipated his own death and named a Council of Five Regents to rule until the child came of age.

Among these men, Tokugawa Ieyasu held the most military strength and political influence, and his authority grew quickly after Hideyoshi’s death in 1598. 

Tension grew as Ieyasu’s rivals began to suspect his intentions. At the imperial court, Ishida Mitsunari organised an alliance of Toyotomi loyalists determined to prevent a Tokugawa takeover.

Supporters of both leaders started to raise armies and prepare for open conflict. 

Eventually, neutral lords were forced to take sides. In the final months before the battle, both factions accused each other of disloyalty and claimed to act in defence of imperial order.

In August 1600, Tokugawa-aligned forces defeated loyalist defenders at the Battle of Gifu Castle, eliminating a key threat to Ieyasu's route west.

The armies finally marched to Sekigahara, where the fate of Japan would be decided in a single day. 

Tokugawa Ieyasu led the Eastern Army, which included skilled commanders such as Ii Naomasa, Honda Tadakatsu, and Date Masamune.

With large, well-equipped armies, they represented some of the most powerful daimyo in eastern and northern Japan.

Ieyasu's total force likely numbered around 75,000 men. 

Among Ieyasu’s allies, many supported his goal of national stability under a stable military government.

Others joined him for more practical reasons, believing his rule would protect their lands and allow them to maintain their status.

Behind their banners stood tens of thousands of ashigaru, samurai, and matchlock gunners ready for combat. 

In contrast, the Western Army gathered around Ishida Mitsunari. His supporters included Toyotomi loyalists such as Ukita Hideie, Konishi Yukinaga, and Otani Yoshitsugu.

Each had gained favour under Hideyoshi’s regime and viewed Ieyasu’s rise as a direct threat to the Toyotomi rule.

Their army may have included as many as 85,000 men, many of them stationed on high ground with shorter supply routes. 

Importantly, not all commanders remained loyal to Mitsunari. Wakisaka Yasuharu and Ogawa Suketada secretly planned to betray Mitsunari once the outcome of the battle became clear. 

Initially, the Western Army appeared to hold the advantage, since their troops occupied high ground, they had superior numbers, and they had shorter supply lines.

However, Ieyasu’s leadership and unified command structure gave the Eastern Army a critical edge. 

At dawn on 21 October 1600, thick fog covered the battlefield and delayed the first manoeuvres.

As the fog lifted, Tokugawa Ieyasu gave the order for his troops to advance. The central line, led by Ii Naomasa, struck Ukita Hideie’s position with force and accuracy.

The fighting would continue into the early afternoon. 

Soon after, a series of shots rang out across the battlefield. Foot soldiers who carried spears and muskets pushed forward into the enemy line.

The fighting grew intense as the units on both sides engaged in close-quarters combat and struggled to gain the upper hand. 

Tokugawa’s other commanders began to stretch the Western flanks. They tested the stability of enemy formations through organised attacks and alternating waves.

Otani Yoshitsugu’s position came under increasing pressure from both the front and the side. 

At the same time, Kobayakawa Hideaki remained inactive on Mount Matsuo. His silence frustrated commanders on both sides, as his support or betrayal would influence the outcome.

In response, Ieyasu ordered his gunners to fire warning shots at Hideaki’s camp, which forced him to make a choice. 

After hours of hesitation, Kobayakawa Hideaki broke his silence. His troops moved down the slope and launched a surprise attack on Otani Yoshitsugu’s right flank.

The betrayal created immediate chaos in the Western ranks and shattered the unity of Mitsunari’s line.

Hideaki, who had previously been scolded by Hideyoshi and felt humiliated in court, likely saw Ieyasu's rise as a chance to restore his standing. 

In the wake of Hideaki’s attack, several other daimyo switched sides. Wakisaka Yasuharu and Ogawa Suketada turned against their former allies and joined the assault.

Otani’s forces fell apart under the weight of attacks from multiple directions. 

As the Western Army unravelled, Shimazu Yoshihiro attempted to withdraw. He launched a fierce breakout with his remaining troops and succeeded in escaping, though many of his men were cut down in the attempt.

Other commanders, unable to organise a retreat, surrendered or fled. 

Eventually, Tokugawa’s forces controlled the field. Soldiers secured enemy banners, gathered abandoned equipment, and took prisoners.

In the following days, Mitsunari was captured at Mount Ibuki and later executed on 6 November 1600, along with many other leaders of the Western faction. 

After his victory, Tokugawa Ieyasu acted quickly to strengthen control. He seized lands from defeated daimyo and awarded them to trusted allies.

As a result, he created a network of loyal lands that surrounded key strategic areas. 

Around major roads and castle towns, Ieyasu stationed his most dependable vassals.

He also strengthened his influence over the imperial court by marrying family members into noble households. 

In 1603, the emperor granted him the title of shogun. With this title, Ieyasu formally established the Tokugawa shogunate.

Soon after, he retired and passed power to his son, though he continued to direct affairs behind the scenes until his death in 1616. 

Under the new government, the shogunate put in place laws that limited the power of the daimyo.

Each lord was required to alternate residence between Edo and their home domains, with their families effectively held as hostages.

This policy, known as the Sankin-kōtai system, drained the resources of regional lords and ensured their obedience through constant watch.

Although Ieyasu laid the groundwork for this approach, it was formalised later in 1635 under his grandson, Tokugawa Iemitsu. 

Foreign missionaries were expelled, foreign trade was restricted, and Christianity was banned.

Although initial restrictions had begun under Toyotomi Hideyoshi, Ieyasu permitted limited missionary activity until 1614, when he issued an edict banning Christianity outright. 

Eventually, Tokugawa rule brought an end to the long period of internal warfare.

The Edo period introduced peace and encouraged growth in population and urban areas.

Samurai became bureaucrats, the merchant class expanded, and strict social rules guided everyday life. 

Ultimately, his victory ended the chaos of the Sengoku period and set the foundations for more than two centuries of Tokugawa rule.