By 200 AD, rice cultivation had been known on the islands east of the Asian continent for 500 years. With agriculture had come ownership of land where previously, boundaries between small nomadic hunting groups had been indistinct. People came to live together in communities, sharing in the work of planting and harvesting, and in defense against others who would take their winter stores. With the possession of land had come war.

The growth of farming drew people away from hunting and away from hunting skills. Some were naturally better suited to fighting then others and so honed those skills, becoming specialists at fighting. Those who fought became warriors, and by virtue of their strength, became the leaders of their clans. The wars they fought resulted in larger clans overcoming and absorbing smaller ones. Japanese society of the third century was composed of many clans, capable and willing to wage war for advantage. It would not be very long before they became one society.
By 200 AD, the Chinese Han court had received envoys from as many as 30 clans from northern Kyushu through their offices on the Korean peninsula. The ancestors of the Japanese had much more reason to look west than to the northern wilderness since the west held much to attract them in both materials and technology. Korean iron and weapons were particularly desirable. Shortly after the fall of the Han in 220 AD, Kyushu clans, capable and willing to wage war for advantage, attacked.
Warriors of this era fought on foot with bows, stabbing swords, and spears. Armor was worn, but most warriors probably had only shields. Steel and bronze had come to the Japanese islands with rice and so they knew of and used these materials. The more advanced technology and the better materials, however, were still from the continent.
By 300 AD the religious, political, and military consolidation of independent clans culminated with the Yamato clan becoming dominant. Included in the consolidation were clans on northern Kyushu and southern Honshu. The Yamato were in power because of the support of many clans rather than the surrender of those clans. The Yamato culturally consolidated early Japanese society; administratively, many local clans remained relatively independent. Archeological excavations show that mound tombs constructed in this time were all very similar and yet widely distributed. They demonstrate the cultural unity of the people, the independence of distributed clans, and the measure of their power over the lower classes. Yamato invasions of the Korean peninsula were frequent, leading even to the establishment a land-hold. The tip of the peninsula, called Mimana, long under the influence of the islanders, was established as their own domain and base for raids in the fourth century. From this presence, the flow of culture and technology was assured.
War with the continentals was not always a matched fight, however. Shortly after 400 AD, the enemy demonstrated that they had learned to fight from horseback. Up until that time, horses, though available, had not been ridden in war by the Yamato. Shooting an arrow from horseback required two hands. Until the invention of stirrups in China in the first century, falling off one's horse was a much more likely event than successfully launching a home hitting arrow. With stirrups providing two-sided support and a saddle to brace one's knees, a warrior could stand, use his feet to guide the horse, shoot arrows, and swing a sword. With the additional speed offered by a mount, foot soldiers could be easily surprised and devastated by many fewer men. The Yamato participated in politics and culture on the Korean peninsula directly. Alliances were made and war waged. Even marriages were arranged between courts. The Yamato and Paikche found a common enemy in the Silla and so allied against them. Paikche and Yamato, as allies, exchanged knowledge and material. Scribes arrived in Yamato almost immediately after contact. Buddhism arrived in 538. Swordsmiths, armorers, and horses all made their way to Yamato. By 600, one third of the Yamato court was composed of foreign immigrants brought to Yamato for their advanced knowledge and skills. In time, the developed skills of the Yamato made their products so desirable that the exchange reversed, and weapons and horses were exported back to the Paekche to aid in the fighting.
Foreign wars were not all that concerned the Yamato, however. Unity in the Yamato court was not the rule. Prior to the 6th century, the Great Lord was the religious and political leader of the nation. (The Yamato ruler had not yet been attributed with divine authority.) The position was hereditary, but without rules for ascension. Each Great Lord kept consorts in great numbers and so it was not always apparent who would reign next. Since power was involved, outside clans often tried to gain influence by marrying daughters to princes in the hope that a son-in-law prince would become Great Lord. To that end, clans would support their sons-in-law by murdering rivals or by waging war on other clans. One clan warrior was so thorough in killing off competitors, that when his chosen prince died shortly after taking office, a wide search found that the only hereditary choice remaining was a prince who had been in hiding and who was patronized by a rival clan.
War on the Korean peninsula eventually led to the expulsion of the Yamato from Mimana in 562. During the continental wars before and after their expulsion, the Yamato, having become skilled as mounted archers, were often called upon to help their allies. The warriors sent to the continent rode horses which were small, perhaps 40" at the shoulders. Armor for the horses was excluded so that they would be quick. Arrows were the weapon of choice. Swords were used from horseback, but most likely only after arrows had been depleted. Armor was therefor designed primarily to repel arrows.
In 663, with support from the Tang dynasty in China, Silla overwhelmed Yamato and Paekche forces in a deafening defeat. The Chinese had brought new tactics to the battlefield for directing mass peasant armies armed with crossbows. With this defeat, Silla went on to unify the peninsula. Yamato, now reeling with the likelihood of invasion from abroad, withdrew to defend itself.
Just as the Yamato began organizing for defense, difficulties at court took precedence over defense. The Great Lord died and in a subsequent civil war, the next leader, Temmu, took power by the military defeat of his brother.
Temmu became the first "Heavenly Warrior Emperor" of Japan. From the experience of losing to the Chinese and from fears of invasion, Temmu established laws to control the military strength of the nation. Having taken the throne by force, he knew especially well that military power meant ruling power. A peasant conscript army was established, with weapons being the possessions of the government. Conscripted service for border guards was required of all clan warriors for periods of three years. Horsemen were to train continuously, peasants 10 out of 100 days. Skills to be practiced included swinging swords, stabbing with spears, firing crossbows, and catapulting stones. "In a government, military matters are the essential thing." stated Temmu.
Temmu also knew the value of diplomacy. Several missions were sent to China, carefully avoiding the Korean peninsula. It was during their first direct contacts with the Tang dynasty that the Yamato first began referring to their islands as the "sun source" or "Nippon". The Chinese pronounced the same characters as "Jihpen". It is this sound which Marco Polo brought back to Europe in the thirteenth century.
The Kanto plain in central Honshu was ideal for raising horses, and perhaps from hunting and military engagements with northern barbarians or "Emishi", the Kanto warriors had long been known as the fiercest of Japan. Eventually, because of their greater skill, border guards came to be Kanto plain warriors almost exclusively. Some of the first written documents available from Japanese history are poems written by warriors about service as border guards:
From today
Without regard for myself
I set out
A shield strong but humble
For our Sovereign Lord.
In time, however, the threat from the continent diminished and raids from the Emishi became a more pressing concern. In the eighth century, conscripts and Kanto mounted warriors were sent to the north to bring the tribes under court control.
The Emishi were fast horsemen, however, and fought a guerrilla style war. Conscript armies were not effective and were often overwhelmed. The wars eventually stretched the financial limits of the government, and with a population decline due to smallpox and crop failures, armies came to consist only of mounted archers. Crossbows for the peasant armies proved too expensive. Also due to expense and to the rusting failure of iron armor, the government turned to leather armor. The benefit of light weight was an added attraction.
By the end of the eighth century, the Japanese warrior fighting in the northern extents of Honshu essentially fit the historical model of the classical Japanese warrior. By this time, the government had come to depend on the men of the Kanto to such an extent that courtiers no longer personally took up arms. The term "samurai", meaning "those who serve" came into use (although with derogatory meaning when used by pretentious courtiers). Warriors continued to fight on horseback, with bow and arrows as their primary weapon, but also with a newly designed sword. The Emishi had been found to be fighting with curved swords. These seemed much better suited to slicing cuts inflicted from horseback, and so the Japanese tachi, which had previously been modeled on the continental sword, was revised for the same effect.