Ashikaga Shogunate and the Feudal Period

Various samurai clans struggled for power during the Kamakura and Ashikaga Shogunates.

Zen Buddhism spread among the samurai in the 13th century and helped to shape their standards of conduct, particularly overcoming fear of death and killing, but among the general populace, Pure Land Buddhism was favored.

In 1274, the Yuan Dynasty of the Mongol Empire sent a force of some 40,000 men and 900 ships to invade Japan in northern Kyūshū. Japan mustered a mere 10,000 samurai to meet this threat. The invading army was harassed by major thunderstorms throughout the invasion, which aided the defenders by inflicting heavy casualties. The Yuan army was eventually recalled and the invasion called off. The Mongol invaders used small bombs, which was likely the first appearance of bombs and gunpowder in Japan.

The Japanese defenders recognized the possibility of a renewed invasion, and began construction of a great, stone barrier around Hakata Bay in 1276. Completed in 1277, this wall stretched for 20 kilometers around the border of the bay. This would later serve as a strong defensive point against the Mongols. The Mongols attempted to settle matters in a diplomatic way from 1275 to 1279, but every envoy sent to Japan was executed. This set the stage for one of the most famous engagements in Japanese history.

In 1281, a Yuan army of 140,000 men with 4,400 ships was mustered for another invasion of Japan. Northern Kyūshū was defended by a Japanese army of 40,000 men. The Mongol army was still on its ships preparing for the landing operation when a typhoon hit north Kyūshū island. The casualties and damage inflicted by the typhoon, followed by the Japanese defense of the Hakata Bay barrier, resulted in the Mongols again recalling their armies.

The thunderstorms of 1274 and the typhoon of 1281 helped the samurai defenders of Japan repel the Mongol invaders despite being vastly outnumbered. These winds became known as kami-no-kaze, which literally translates as "wind of the gods." This is often given a simplified translation as "divine wind." The kami-no-kaze lent credence to the Japanese belief that their lands were indeed divine and under supernatural protection.

In the 14th century, a blacksmith called Masamune developed a two-layer structure of soft and hard steel for use in swords. This structure gave much improved cutting power and endurance, and the production technique led to Japanese swords (katana) being recognized as some of the most potent hand weapons of pre-industrial East Asia. Many swords made using this technique were exported across the East China Sea, a few making their way as far as India.

Issues of inheritance caused family strife as primogeniture became common, in contrast to the division of succession designated by law before the 14th century. To avoid infighting, invasions of neighboring samurai territories became common and bickering among samurai was a constant problem for the Kamakura and Ashikaga Shogunates.

The Sengoku jidai ("warring-states period") was marked by the loosening of samurai culture with people born into other social strata sometimes making names for themselves as warriors and thus becoming de facto samurai. In this turbulent period, bushido ethics became important factors in controlling and maintaining public order.

Japanese war tactics and technologies improved rapidly in the 15th and 16th century. Use of large numbers of infantry called ashigaru ("light-foot," due to their light armour), formed of humble warriors or ordinary people with Nagayari (a long lance) or (Naginata), was introduced and combined with cavalry in maneuvers. The number of people mobilized in warfare ranged from thousands to hundreds of thousands.

The arquebus, a matchlock gun, was introduced by the Portuguese via a Chinese pirate ship in 1543 and the Japanese succeeded in assimilating it within a decade. Groups of mercenaries with mass-produced arquebuses began playing a critical role.

By the end of feudal period, several hundred thousand firearms existed in Japan and massive armies numbering over 100,000 clashed in battles. By comparison, the largest and most powerful army in Europe, the Spanish, had only several thousand firearms and could only assemble 30,000 troops.

In 1592, and again in 1598, Toyotomi Hideyoshi decided to invade China (唐入り) and sent to Korea an army of 160,000 samurai (Hideyoshi's invasions of Korea, 朝鮮征伐). Taking advantage of its mastery of the arquebus, Japanese samurai almost led the war to victory, but were unable to do so, due to the entry of Ming Chinese troops. A few of the more famous samurai generals of this war were Katō Kiyomasa, Konishi Yukinaga, and Shimazu Yoshihiro.

Social mobility was high, as the ancient regime collapsed and emerging samurai needed to maintain large military and administrative organizations in their areas of influence. Most of the samurai families that survived to the 19th century originated in this era, declaring themselves to be the blood of one of the four ancient noble clans, Minamoto, Taira, Fujiwara and Tachibana. In most cases, however, it is hard to prove these claims.