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The Battle of Sekigahara marked the turning point from the Azuchi-Momoyama period to the Edo period.
This battle enabled Tokugawa Ieyasu to unify Japan, ushering in the Edo period.
So why did the Battle of Sekigahara happen?
Here is a brief and easy-to-understand explanation.
(Some expressions are deliberately exaggerated for clarity)
Toyotomi Hideyoshi fell gravely ill around the age of 60. On his deathbed, he summoned Tokugawa Ieyasu.
Toyotomi Hideyoshi: “When I die, I will pass the rule of Japan to you.”
Tokugawa Ieyasu: “Please, do not say such things. You have your son Hideyori. Please pass the succession to Lord Hideyori, not to me.”
Toyotomi Hideyoshi: “Hideyori is only 5 years old and too young. Please serve as his guardian and handle government affairs until he comes of age.”
Tokugawa Ieyasu: “I understand.”
After this exchange, Toyotomi Hideyoshi passed away.
As Hideyoshi wished, his son Toyotomi Hideyori inherited the realm. Along with this, the Five Commissioners and Five Elders system was established, where these officials would support the young Hideyori and govern the nation.
The Five Commissioners were Hideyoshi’s most capable administrators, while the Five Elders were the most powerful feudal lords in Japan.
| Name | Romanization | Notes |
|---|---|---|
| Ishida Mitsunari | Ishida Mitsunari | Chief Commissioner |
| Asano Nagamasa | Asano Nagamasa | |
| Maeda Gen - i | Maeda Gen - i | |
| Mashita Nagamori | Mashita Nagamori | |
| Natsuka Masaie | Natsuka Masaie |
| Name | Romanization | Notes |
|---|---|---|
| Tokugawa Ieyasu | Tokugawa Ieyasu | Chief Elder |
| Maeda Toshiie | Maeda Toshiie | |
| Ukita Hideie | Ukita Hideie | |
| Mori Terumoto | Mori Terumoto | |
| Kobayakawa Takakage | Kobayakawa Takakage | Replaced by Uesugi Kagekatsu after his death in 1597 |
However, under this system, Tokugawa Ieyasu—the head of the Five Elders—steadily accumulated power and began seizing control. This displeased Ishida Mitsunari and the other commissioners, leading to conflict between them and Ieyasu.
Ishida Mitsunari, as Chief Commissioner, had been managing the practical affairs of the Toyotomi government and was most wary of Ieyasu’s growing influence.
This conflict ultimately led to the Battle of Sekigahara.
The Battle of Sekigahara was not a pre-arranged confrontation. Rather, it resulted from escalating tensions between both sides. Here are the key incidents that made war inevitable.
Ishida Mitsunari once narrowly escaped an assassination attempt. The attackers were the Seven Spears of Shizugatake—generals who simply held personal grudges against Mitsunari.
However, rumors began circulating that Tokugawa Ieyasu was secretly behind the plot.
Toyotomi Hideyoshi had prohibited political marriages between feudal lords, among other things. However, Tokugawa Ieyasu ignored these rules and did as he pleased. This constantly infuriated Ishida Mitsunari.
To criticize Ieyasu’s disregard for the rules, Naoe Kanetsugu—an ally of Mitsunari—sent a letter in response to Ieyasu’s demands for an explanation.
This letter enraged Tokugawa Ieyasu. (His anger was reportedly due to the disrespectful tone toward a superior.)
Following this, both sides mobilized their armies, and the Battle of Sekigahara began.
Tokugawa Ieyasu: “Ishida Mitsunari is a nuisance. By the way, you are on my side, right?”
Ishida Mitsunari: “Tokugawa Ieyasu is dangerous. By the way, you are on my side, right?”
In the lead-up to the Battle of Sekigahara, every feudal lord was forced to choose between the Western Army (led by Ishida Mitsunari) or the Eastern Army (led by Tokugawa Ieyasu).
Kuroda Nagamasa, who later distinguished himself at Sekigahara, had spent his childhood under Toyotomi Hideyoshi and had close ties to the Toyotomi clan. Based on this alone, joining the Western Army to support Hideyori would have been natural. However, due to policy disagreements with Ishida Mitsunari, he ultimately sided with the Eastern Army.
Thus, each feudal lord’s choice between the Western and Eastern armies involved complex calculations of self-interest, self-preservation, and personal relationships. For example, Kobayakawa Hideaki initially joined the Western Army but defected to the Eastern Army during the battle itself. It is said that a secret agreement had been made: even if the Eastern Army won, his lands would not be confiscated.
The Battle of Sekigahara was fought in 1600, after Toyotomi Hideyoshi’s death, between Tokugawa Ieyasu’s Eastern Army and the Western Army centered around Ishida Mitsunari.
Ishida Mitsunari had served as Hideyoshi’s close aide and became one of the most influential figures in the Toyotomi government. Mitsunari sought to honor Hideyoshi’s dying wish and establish Hideyori as the future ruler of Japan.
Meanwhile, Tokugawa Ieyasu expanded his power after Hideyoshi’s death, aiming to become the supreme ruler himself. Ieyasu sought to suppress Mitsunari’s authority and strengthen his own position.
Fearing Ieyasu’s growing power, Ishida Mitsunari resolved to raise an army and eliminate him. Mitsunari believed that by removing Ieyasu, he could secure Hideyori’s position as the rightful ruler.
Rallying the feudal lords of western Japan to his cause, Ishida Mitsunari clashed with Ieyasu’s Eastern Army at Sekigahara on September 15, 1600.
The Western Army numbered approximately 80,000 troops against the Eastern Army’s roughly 100,000. Due to this numerical disadvantage and betrayals within the Western ranks, the battle was decided in just six hours.
The Eastern Army emerged victorious. Ishida Mitsunari was captured and executed.
Through this victory, Ieyasu became the supreme ruler and went on to establish the Tokugawa shogunate.
The Battle of Sekigahara marked the end of the Sengoku period and the beginning of the Edo era, making it one of the most significant battles in Japanese history.
Many feudal lords and warriors were involved, and researching why each chose the Eastern or Western side makes for fascinating study.
