stylistic:tone
Sengoku period

The outbreak of the Onin War, which began in 1467 and lasted about 10 years, devastated Kyoto and brought down the power of the Ashikaga Shogunate. This plunged the country into the Sengoku Period ("warring states period"), in which daimyo (feudal lords) from different regions fought each other. This period corresponds to the late Muromachi period. There are about nine theories about the end of the Sengoku Period, the earliest being the year 1568, when Oda Nobunaga marched on Kyoto, and the latest being the suppression of the Shimabara Rebellion in 1638. Thus, the Sengoku Period overlaps with the Muromachi, Azuchi–Momoyama, and Edo periods, depending on the theory. In any case, the Sengoku period was a time of large-scale civil wars throughout Japan.

Daimyo who became more powerful as the shogunate's control weakened were called sengoku daimyo (戦国大名), and they often came from shugo daimyo, Shugodai (守護代, deputy Shugo), and kokujin or kunibito (国人, local masters). In other words, sengoku daimyo differed from shugo daimyo in that sengoku daimyo was able to rule the region on his own, without being appointed by the shogun.

During this period, the traditional master-servant relationship between the lord and his vassals broke down, with the vassals eliminating the lord, internal clan and vassal conflicts over leadership of the lord's family, and frequent rebellion and puppetry by branch families against the lord's family. These events sometimes led to the rise of samurai to the rank of sengoku daimyo. For example, Hōjō Sōun was the first samurai to rise to the rank of sengoku daimyo during this period. Uesugi Kenshin was examples of Shugodai who became sengoku daimyo by weakening and eliminating the power of their lords.

This period was marked by the loosening of samurai culture, with people born into other social strata sometimes making a name for themselves as warriors and thus becoming de facto samurai. One such example is Toyotomi Hideyoshi, a well-known figure who rose from a peasant background to become a samurai, sengoku daimyo, and kampaku (Imperial Regent).

From this time on, infantrymen called ashigaru, who were mobilized from the peasantry, were mobilized in even greater numbers than before, and the importance of the infantry, which had begun in the Nanboku-chō period, increased even more. When matchlocks were introduced from Portugal in 1543, Japanese swordsmiths immediately began to improve and mass-produce them. The Japanese matchlock was named tanegashima after the Tanegashima island, which is believed to be the place where it was first introduced to Japan. By the end of the Sengoku Period, there were hundreds of thousands of arquebuses in Japan and a large army of nearly 100,000 men clashing with each other.

On the battlefield, ashigaru began to fight in close formation, using yari (spear) and tanegashima. As a result, yari, yumi (bow), and tanegashima became the primary weapons on the battlefield. The naginata, which was difficult to maneuver in close formation, and the long, heavy tachi fell into disuse and were replaced by the nagamaki, which could be held short, and the short, light katana, which appeared in the Nanboku-cho period and gradually became more common. The tachi was often cut off from the hilt and shortened to make a katana. The tachi, which had become inconvenient for use on the battlefield, was transformed into a symbol of authority carried by high-ranking samurai. Although the ōdachi had become even more obsolete, some sengoku daimyo dared to organize assault and kinsmen units composed entirely of large men equipped with ōdachi to demonstrate the bravery of their armies.

These changes in the aspect of the battlefield during the Sengoku period led to the emergence of the tosei-gusoku style of armor, which improved the productivity and durability of armor. In the history of Japanese armor, this was the most significant change since the introduction of the ō-yoroi and dō-mal in the Heian period. In this style, the number of parts was reduced, and instead armor with eccentric designs became popular.

By the end of the Sengoku period, allegiances between warrior vassals, also known as military retainers, and lords were solidified. Vassals would serve lords in exchange for material and intangible advantages, in keeping with Confucian ideas imported from China between the seventh and ninth centuries. These independent vassals who held land were subordinate to their superiors, who may be local lords or, in the Edo period, the shogun. A vassal or samurai could expect monetary benefits, including land or money, from lords in exchange for their military services.