Ashikaga Shogunate and the Feudal Period

In the 13th century, Yuan, a Chinese state of the Mongolian Empire, invaded Japan twice. Samurai unused to fighting in group barely survived the first brief battle. However, they were prepared for the second invasion by building a defensive stone wall on the Mongols' landing shore, and adopting a night attack tactic. Overall, the Samurai way of warfare was incapable of inflicting significant damage upon the Mongol army, which favored tactics of large encirclement, blitzkrieg, and employed advanced weaponary (Samurais were shocked by the Chinese grenades). In the end, it was the second typhoon that destroyed the Mongol armada, and prevented Yuan Dynasty from annexation of Japan. Japanese deemed the typhoon "the divine wind" or "kamikaze" in Japanese. Two major military elements were acquired from Mongol invasions: 1) the importance of infantry and 2) the weakness of Japanese longbows and of the conventional Samurai cavalry against the invaders. As the result of this, Samurais gradually replaced the way of bow with the way of "blades". At the beginning of 14th century, swords and spears became the mainstream among Japanese samurai warlords. An innovation on Japanese sword was produced by a blacksmith called Masamune in the 14th century; the two-layer structure of soft and hard iron was adopted and the style spread rapidly with its amazing cutting power and endurance in continuous use. Since then, Japanese swords had been recognized as one of the most potent hand weapons during the pre-industrial era of East Asia. It was one of the top exported items, a few even making their way as far as India.

The issues of inheritance caused family infighting, because primogeniture became common, while division of succession was designated by law before the 14th century. To avoid infighting, continuous invasion against neighboring samurai's' territories was rather favored and bickering among samurai was constant problems to Kamakura and Ashikaga Shogunate.

The Sengoku jidai ("warring-states period") was marked by the loosening of samurai culture, in a sense. Those born into other social strata could sometimes make names for themselves as warriors and thus become de facto samurai. In this turbulent period, bushido ethics became important factors to control and maintain public orders.

Japanese war tactics and technologies improved rapidly in 15th and 16th century. Use of large infantry troop called Ashigaru ("light-foot", due to their light armour), which was formed by the humble warriors or populace, with Nagayari or long lance was introduced and combined with cavalry in maneuvers. The numbers of people mobilized in warfare were generally in the thousands to the over hundred-thousands.

Harquebus or a matchlock gun was introduced by Lusitanians/Portuguese on a Chinese pirate ship in 1543. Japanese succeeded nationalization of it within a decade. Groups of mercenaries with harquebus and mass produced rifles played a critical role. By the end of feudal periods, several hundred thousands rifles existed in Japan and massive armies over 100,000 clashed in the battles. The largest and most powerful army in Europe, the Spanish armies, had only several thousand rifles and could only assemble an army of 30,000. Ninja also played critical roles engaged in intelligence activity.

The social mobility of human resources was flexible, 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. They declared 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 who their ancestors were.

Oda, Toyotomi and Tokugawa