The Unification of Japan: A Century of Turmoil

After the unification of Japan, the country went through a 250-year period of peace — but before that, it suffered almost a century and a half of constant internecine conflict.

Dec 14, 2022By Michael Smathers, BA HIstory w/ focus on Medieval Periods

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The Sengoku Jidai, or the Age of Warfare, was Japan’s most dynamic and chaotic period. Dozens of daimyo, or feudal lords, fought incessantly for control of the capital Kyoto, and with it control of the country. So how did this state of affairs start and how did it play out to ultimately result in the unification of Japan? In this article, we’ll touch upon the most notable events of the time period.


Before the Unification of Japan: The Onin War

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Great Battle from the Record of the Onin War, by Utagawa Yoshitora, 1847, via Ukiyo-e


This upheaval began, as many feudal political situations did, because of a crisis of succession. For two centuries, the Ashikaga shogunate had ruled Japan. The latest shogun, however, Ashikaga Yoshimasa, lacked an heir. He went to his brother Yoshimi, who agreed to leave his life as a monk and take power.


In 1465, Yoshimasa finally sired a son, Yoshihisa. Naturally, this led to a quandary over who would rule. On one side was the shogun’s newly-appointed brother/heir and the clan of his deputy Hosokawa Katsumoto. On the other, was his son, his son’s mother and self-appointed regent Hino Tomiko, and one of the most powerful supporting clans, the Yamana.


A mansion on each side got burned (among various other tensions), setting the two armies at war, in a conflict that lasted for nearly ten years. The shogun, not to put too fine a point on the issue, played the shamisen while Kyoto burned and the warring families bled each other dry to no avail. Seeing that no one would stop them and that the shogun was powerless, all the daimyo of Japan took carte blanche to settle their various quarrels at swordpoint and jockey for position to rule Kyoto.

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The Great Clans of Note

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Troops of Uesugi Kenshin and Takeda Shingen, by Utagawa Sadahide, 1848, via Ukiyo-e


From the end of the Onin War in 1477 until 1543, there was a relative status quo of infighting. Clans rose and fell. This was a time of the greatest social mobility in Japanese history, where peasants could become daimyo or vice versa. Although hundreds of clans are recorded as having existed, several stand out as the most powerful and lasting: the Takeda, Uesugi, Hojo, Oda, Imagawa, Mori, and Shimazu.


The Hojo descended from a family that had been a part of the Imperial Court and the shogunate centuries ago. They lived in the Kanto region around Edo (present-day Tokyo). The Takeda and Uesugi dominated the northeast of Honshu, the main island of Japan — and their clan rivalry is legendary in Japanese history. The Mori controlled the western part of Honshu. Shimazu occupied the western island of Kyushu, and the Oda was just west of the Hojo’s domain.


These were the general areas that each of these clans controlled; given the chaos of the period, their spheres of influence shifted constantly before the unification of Japan.


Tanegashima and the Beginnings of European Influence

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Fuji Seen in the Distance/Samurai Procession with Muskets, by Katsushika Hokusai, 1830, via Ukiyo-e


Until 1543, Japan had had no major diplomatic or trade relations with countries other than China. However, in this year, a Portuguese ship took shelter just off the island of Tanegashima. This ship contained a collection of matchlock rifles. The daimyo of the area, Tanegashima Tokitaka, bought two of them, kept one, and had a swordsmith study and duplicate the firing mechanism and barrel. After that, Japan took to manufacturing and buying these rifles with great enthusiasm.


A few years later, in 1549, Frances Xavier, a Portuguese Jesuit priest, arrived in Japan to spread Catholicism. Portugal controlled most of the trade networks in this part of the world, and although the Japanese knew how to make tanegashima (matchlock guns), the various daimyo had to resort to trading to get gunpowder. The daimyo who converted to Catholicism found it easier and cheaper to get these resources.


The Battle of Okehazama: Oda Nobunaga’s Ascent

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Oda Nobunaga of Owari Province, by Utagawa Yoshitora, 1846, via Waseda University


“If the cuckoo does not sing, kill it.” — (Japanese proverb concerning Oda Nobunaga)


In 1560, the daimyo Imagawa Yoshimoto saw his opportunity to strike at the heart of the country and usher in the unification of Japan. He would lead his forces up the Tokaido road. This path went through the Owari province, recently unified under Oda Nobunaga. Imagawa, who had control over three provinces, expected to tear through Owari like a storm. Along with his allies in the Matsudaira clan, their forces numbered about 25,000.


Indeed, the few forts on the outskirts of the province fell quickly. The Oda army was less than 3,000, but if military history has shown anything, numbers are no guarantee of victory. The Imagawa-Matsudaira forces found themselves in a narrow heavily-forested valley near the Zenko-ji shrine. Nobunaga positioned flags and banners to give the illusion of a larger force, had his men flank the opposing army under cover of a thunderstorm, and attacked from the rear.


Imagawa’s forces were caught off guard and routed, and Yoshimoto was killed. The weakened clan was ripped to shreds and Imagawa vassals including the Matsudaira, pledged themselves to Nobunaga because he had demonstrated his brutal brand of effectiveness. One Matsudaira lord, Takechiyo, renamed himself Tokugawa Ieyasu. This name would be important later in history.


The Ikko-Ikki and the Honno-ji Incident

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Through Fire, Nobunaga Attempting to Escape Honno-ji, by Toyohara Chikanobu, 1898 via Ukiyo-e


Feudal society, whether in Europe, Japan, or elsewhere, was a harsh and cruel life. Starvation, sickness, and toil were often the lot of peasants. Nobility lived in better conditions but their lord could call them up to fight, where they would die horrifically in battle. Peasants sometimes rose up against their overlords. The Ikko-ikki were one such group, led by the sohei, a sect of fiercely devout warrior monks who would not submit to samurai rule.


Over the last two decades since his victory at Okehazama, Nobunaga had taken over central Japan without effective resistance because of his unconventional tactics and his use of ashigaru levies armed with arquebuses. Even the Takeda clan had been effectively wiped out at Nagashino in 1575. Oda Nobunaga was widely known for his ruthlessness, and he demonstrated this when he attacked Enryaku-ji, the most prominent Buddhist temple in Japan. All the Ikko-ikki were put to the sword to remove the threat to the unification of Japan.


Unfortunately for Nobunaga, he didn’t read up on his Machiavelli: in the process of sowing fear among the populace, it is important to not become hated. One of Nobunaga’s generals, Akechi Mitsuhide, was a devout Buddhist and he resented his lord for these atrocities. Nobunaga also frequently bullied and humiliated Mitsuhide. He moved to the west to assist another of the generals, Toyotomi Hideyoshi, in his campaigns against the Mori clan. Finding out that Nobunaga was resting at the Honno-ji shrine, Mitsuhide saw his opportunity and lit the shrine on fire, proclaiming, “The enemy is at Honno-ji.” Nobunaga and his eldest son, failing to escape, committed seppuku that evening: June 21, 1582.


Hideyoshi’s Revenge and Conquest

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Shizu Peak Moon — Hideyoshi, by Tsukioka Yoshitoshi, 1888, via Ukiyo-e


“If the cuckoo does not sing, persuade it.” — (Japanese proverb concerning Hideyoshi)


Toyotomi Hideyoshi was one of Nobunaga’s most loyal generals, having been promoted through the ranks on the basis of merit. At the time of Nobunaga’s death, Hideyoshi was besieging Takamatsu Castle and threatening to divert a nearby river and flood the castle. He heard the news of his master’s murder and opted to change the attack plan. This meant approaching the warden of the castle and saying (paraphrased), “I have no time for this, so you can continue this siege and have all your people drown, or you can surrender this castle and your life, and your people can live.”


The warden, one Shimizu Muneharu, surrendered. Peace, and later alliance, developed between the Mori and the Toyotomi loyalists. Hideyoshi promptly marched his army toward the capital and caught up with Mitsuhide’s army at Tenno-yama in the province of Yamazaki. Not being prepared for such a sudden attack, they were routed. It had been only thirteen days since Nobunaga had been killed. Mitsuhide himself fled and most historical accounts say he was slain by disgruntled peasants.


Hideyoshi went on to expand his territory further until he came up against Tokugawa Ieyasu. They met before the battle, and Ieyasu backed down, sensing this was not the time to wage war. Ieyasu instead became a vassal of Hideyoshi, with the privilege of a one-time allowance to decline joining any of his military campaigns. It wasn’t long before Hideyoshi managed to subdue the rest of Japan. The powerful eastern daimyo, including Date Masamune, would later prove instrumental to the unification of Japan.


Attack on Korea

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The Conquest of Korea, by Utagawa Yoshitora, 1863, via Ukiyo-e


By 1592, Japan had been, for the most part, pacified, partially because of the so-called “Sword Hunt”. Hideyoshi issued orders for all weapons to be confiscated from the peasantry, the idea being that only samurai would be allowed to possess them. His ambition turned westward: now that Japan was under one banner, they could attempt to take over Ming China, but to do that, they would have to go through Korea and use Korea as a staging ground.


Hideyoshi himself did not lead the attack. Most of the western daimyo sailed to Korea with their troops; because of their lack of knowledge of the land, poor command decisions, and the tactical brilliance of Korean admiral Yi-sun Shin, the campaign was doomed to fail. The Japanese withdrew, having wasted thousands of lives and untold resources.


Remember how Ieyasu had his one opportunity to refuse to join his lord? This is the time he called in that favor, even though his own forces might have had an effect. Most of the western daimyo, who had lost money and men, were accustomed to being rewarded after battle, but there was no reward coming because the Korean campaign had been a near-complete failure even after the initial success.


The Five Regents

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Portrait of Toyotomi Hideyoshi, 1598, via Osaka City Museum of Fine Arts


In 1598, Toyotomi Hideyoshi died. He left behind a son, Hideyori, who was at the time too young to take up rulership and complete the unification of Japan. To avert the succession crisis that had started the Sengoku period, Hideyoshi had appointed five of the most powerful daimyo to rule together until Hideyori grew up and could assume the role he was born into. The daimyo were picked to keep one another in check. One of them was Tokugawa Ieyasu, who had plenty of ambition of his own, though he was patient enough to bide his time. Through strategic alliances and careful planning, he was also by far the most powerful daimyo of the eastern provinces.


Sekigahara: The Final Battle

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Great Battle of Sekigahara, by Tsukioka Yoshitoshi, 1868, via Ukiyo-e


“If the cuckoo does not sing, wait for it to sing.” — (Japanese proverb concerning Ieyasu)


Unfortunately for Ieyasu, his rivals feared he was getting too powerful. Coupled with the rumors that Ieyasu was behind the growing dissent in the western provinces, a coalition of provinces led by Ishida Mitsunari took this as a provocation to march on Ieyasu’s domain while he was dealing with one of the other regents, Uesugi Kagekatsu. This coalition was called the Western Army.


Ieyasu split his forces — the Eastern Army — in two. Half would take the coastal Tokaido Road; the other half would take the Nakasendo Road to approach Kyoto from the north. Mitsunari, meanwhile, was delayed trying to take Fushimi Castle, an important strategic chokepoint. This siege is Japan’s version of the battle of Thermopylae: 2,000 defenders under Torii Mototada held out against 40,000 for 11 days before dying to the last man.


The armies converged on the field of Sekigahara, near Gifu Castle, on the morning of October 21, 1600. There had been heavy rain recently, and the field was so foggy that the armies literally collided with one another and retreated in a panic. The next four hours saw an initial advantage to the Western Army, who had 125,000 troops to the Eastern Army’s 85,000. However, Ieyasu had received secret dispatches from several commanders in the Western Army promising to defect. Kobayakawa Hideaki, one of these daimyo, was the most important. His forces were positioned atop a hill to charge Ieyasu’s southern flank. They defected at quite possibly the last moment, attacking Mitsunari instead. The Western Army broke and was defeated.


The Aftermath: The Unification of Japan

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Edo Period Theater Scene, Kiyotsune, 1765, via the Library of Congress


By the time the dust had settled, Ieyasu was the undisputed ruler, having completed the unification of Japan. The Tokugawa would reign for the next 250 years, with the country in almost total isolation from the rest of the world. Trade at Nagasaki with the Dutch and Portuguese was still allowed. Aside from the brief siege of Osaka Castle in 1615, the fighting was over.


The three quotes listed above, show how each of the Three Great Unifiers, as they’re called, contributed to the unification of Japan: Nobunaga’s ferocity, Hideyoshi’s persuasiveness, and Ieyasu’s patience.


The defeated West, on the other hand, would nurse their grudges until the Meiji period, rising up in rebellion to overthrow the shogunate.

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By Michael SmathersBA HIstory w/ focus on Medieval PeriodsI am an avid student of history with a focus on medieval periods, specifically the Kamakura period of Japan. I am four years into a BA in history at the University of West Georgia. I also study various martial arts disciplines and have an interest in ancient mythologies.