Kamakura Shogunate

Establishment
Before the establishment of the Kamakura shogunate, civil power in Japan was primarily held by the ruling emperors and their regents, typically appointed from the ranks of the imperial court and the aristocratic clans that vied there. Military affairs were handled under the auspices of the civil government. However, after defeating the Taira clan in the Gempei War, Minamoto no Yoritomo seized certain powers from the aristocracy in 1185 and was given the title of shogun in 1192. The system of government he established became formalized as the shogunate.

The Hōjō Regency
After Yoritomo's death, Hōjō Tokimasa, his widow Hōjō Masako's clan chief and former guardian of Yoritomo, claimed the title of regent (Shikken) to Yoritomo's son Minamoto no Yoriie, eventually making that claim hereditary to the Hōjō clan. The Minamoto remained the titular shoguns, with the Hōjō holding the real power.

With the Regency, what was already an unusual situation became even more anomalous when the Hōjō usurped power from those who had usurped it from the Emperor in the first place. The new regime nonetheless proved to be stable enough to last a total of 135 years, 9 shoguns and 16 regents[1].

With Sanetomo's death in 1219, his mother Hōjō Masako became the Shogunate's real center of power[1]. As long as she was alive, regents and shoguns would come and go, while she stayed at the helm. Since the Hōjō family didn't have the rank to nominate a shogun from among its members, Masako had to find a convenient puppet[2]. The problem was solved choosing Kujo Yoritsune, a distant relation of the Minamoto, who would be the fourth shogun and figurehead, while Hōjō Yoshitoki would take care of day-to-day business[2]. However powerless, future shoguns would always be chosen from either Fujiwara or imperial lineage to keep the bloodline pure[2] and give legitimacy to the rule. This was to become the normal way of doing things for more than a century[2].

In 1221 Emperor Go-Toba tried to regain power in what would be called the Jōkyū War (承久の乱, Jōkyū no Ran?), but the attempt failed[3]. The power of the Hōjō was thereafter unchallenged until 1324, when Emperor Go-Daigo orchestrated a plot to overthrow them, but which however was discovered immediately[1].

Mongol invasions and decline
The Mongols under Kublai Khan attempted sea-borne invasions in 1274 and 1281 (see Mongol invasions of Japan). The Kamakura shogunate met the invaders with vast armies of defenders. With the aid of typhoons, which came to be called "kamikaze," the Mongols were repelled. Many times the mongols were defeated by violent storms, which smashed their ships, although the mongol troops made it to shore they were defeated. However, the strain on the military and the financial expenditures weakened the regime considerably. Additionally, the defensive war left no gains to distribute to the warriors who had fought it, leading to discontent. Construction of defensive walls added further expenses to the strained regime.

In 1331 Emperor Go-Daigo took arms against Kamakura, but was defeated by Kamakura's Ashikaga Takauji and exiled to Oki Island, in today's Shimane Prefecture[3]. A warlord then went to the exiled Emperor's rescue and in response the Hōjō sent forces again commanded by Ashikaga Takauji to attack Kyoto[3]. Once there, however, Ashikaga decided it was time to switch sides, and support the Emperor[3]. At the same time another warlord loyal to the Emperor, Nitta Yoshisada, attacked Kamakura and took it[1]. About 870 Hōjō samurai, including the last three Regents, committed suicide at their family temple, Tōshō-ji, whose ruins were found in today's Ōmachi[1]. Ashikaga in 1336 assumed the position of shogun himself, establishing the Ashikaga shogunate.