Japan’s century of civil war produced the country’s most iconic warriors, its decisive transformation in military technology, and the political settlement that would endure for two and a half centuries. The Sengoku period (戦国時代, sengoku jidai) — “age of warring states” — ran from the collapse of central authority in 1467 to the destruction of the last rival to Tokugawa rule in 1615.

What Caused the Sengoku Period

The immediate trigger was the Ōnin War (応仁の乱, 1467–1477): a succession dispute within the Ashikaga shogunate that brought two armies into Kyoto and left the city largely in ruins. When the war formally ended in 1477, it resolved nothing. Provincial lords (shugo daimyō) who had come to the capital with their forces returned home — or did not return at all — and began consolidating power in their own domains. [history]

Underlying causes were structural. The Ashikaga shoguns (ruling from Kyoto since 1336) had never held the centralised tax base or military monopoly of the later Tokugawa. Authority depended on managing coalitions of semi-independent lords. Once those lords saw weakness at the centre, the coalition unravelled. [history]

By the early sixteenth century, a new type of lord had emerged: the sengoku daimyō, men who had risen by conquest rather than by inheritance from the old court aristocracy. Many were gekokujō (下剋上) figures — literally “low overcoming the high” — men of middling birth who had displaced established houses. [history]

The Three Unifiers

Historians group the resolution of the Sengoku period around three successive figures, sometimes called the “Three Unifiers” (三英傑, san eiketsu).

Oda Nobunaga (1534–1582)

Nobunaga began as a minor lord in Owari province. His victory at Okehazama (1560) — a surprise attack on the numerically superior army of Imagawa Yoshimoto — established his reputation. He entered Kyoto in 1568 backing a puppet Ashikaga shogun, then deposed that shogun in 1573, ending the Muromachi shogunate. [history]

Nobunaga’s military innovations were systematic. He recruited ashigaru (foot soldiers) on merit rather than lineage, deployed firearms in disciplined formations, and developed logistical systems that allowed sustained campaigns. At Nagashino (1575) his arquebusiers — the figures most cited in popular accounts range from 1,000 to 3,000 — broke the Takeda cavalry charge. Modern historian Jeroen Lamers cautions that the famous “three-volley rotation” may be exaggerated in later sources, but the firearm advantage was real. [history] [legend: exact troop numbers from Shinchō-kōki are chronicle figures not independently verified]

Nobunaga was assassinated in June 1582 at Honnō-ji temple in Kyoto by his own general, Akechi Mitsuhide. The motive remains debated. Within eleven days, Toyotomi Hideyoshi had defeated and killed Mitsuhide at Yamazaki. [history]

Toyotomi Hideyoshi (1537–1598)

Hideyoshi came from peasant stock — the only one of the three unifiers not born into the warrior class. He rose through Nobunaga’s service on ability alone and completed the military unification of Honshu with the Siege of Odawara (1590), destroying the Hōjō clan, the last major holdout. [history]

Hideyoshi then launched two invasions of Korea (1592 and 1597) with the stated aim of conquering China. Both failed: Korean naval resistance led by Admiral Yi Sun-sin, Ming Chinese intervention, and Hideyoshi’s death in 1598 ended the campaigns. The invasions left lasting damage to Japanese-Korean relations and drained Hideyoshi’s regime. [history]

Hideyoshi died before he could secure the succession of his young son Hideyori. His council of regents fragmented almost immediately. [history]

Tokugawa Ieyasu (1543–1616)

Ieyasu had been Nobunaga’s ally and Hideyoshi’s subordinate. After Hideyoshi’s death, he manoeuvred against the remaining Toyotomi loyalists. The issue was settled at the Battle of Sekigahara (21 October 1600): the largest land battle in Japanese history by most estimates, fought in central Honshu between Ieyasu’s “eastern” coalition and a “western” coalition nominally defending the Toyotomi heir. [history]

Ieyasu’s side won decisively, in part due to the defection of Kobayakawa Hideaki at a critical moment. In 1603 Ieyasu received the title of Sei-i Taishōgun from the emperor and founded the Edo shogunate. The final act came in 1615 when Ieyasu destroyed Osaka Castle and the Toyotomi line. [history]

Firearms and the Transformation of War

The tanegashima matchlock musket arrived in Japan in 1543 when Portuguese traders landed on Tanegashima island. Within two years Japanese smiths were producing copies. The weapon spread rapidly because it required far less training than the bow and could penetrate armour. [history]

By the 1560s, some daimyō armies fielded thousands of arquebusiers. The logistical and tactical consequences were profound: fortifications had to be rethought, cavalry charges became more dangerous, and massed infantry formations became decisive. The Sengoku period was arguably the first sustained use of firearms in Asian warfare at this scale. [history]

Religion and the Sengoku Period

The Jesuit Francis Xavier arrived in Kagoshima in 1549. Christianity spread quickly in western Kyushu and attracted some daimyō who saw commercial benefits from Portuguese trade connections. By the 1580s, there were an estimated 150,000 Japanese Christians. [history]

Nobunaga tolerated and in some cases actively supported Christian missionaries, partly as a counterweight to Buddhist temple power. Hideyoshi’s attitude shifted: he issued a decree expelling missionaries in 1587 (enforced inconsistently) and crucified twenty-six Christians in Nagasaki in 1597. [history]

The relationship between Buddhist institutions and Sengoku rulers was complex. Many daimyō patronised temples; Nobunaga was an exception in attacking them — most notoriously burning Enryaku-ji on Mt. Hiei in 1571 and reportedly killing thousands. Sources for the death toll vary enormously and must be treated with caution. [history] [legend: casualty figures for Enryaku-ji are unverifiable from surviving records]

The End of the Sengoku Period

The conventional end date is 1615 — the destruction of Osaka Castle and the elimination of Toyotomi Hideyori. Some historians prefer 1603 (Ieyasu’s appointment as shogun) or 1600 (Sekigahara) as the effective end. The 1615 date captures the moment when no armed rival to Tokugawa rule remained. The Edo period that followed would last until 1868 — the longest period of sustained peace in Japanese history.