How did a century of civil war tear Japan apart?

A century of civil war tore Japan apart because the shogunate that was supposed to keep the lords in line lost its grip, and the lords kept fighting once no one could stop them. The Sengoku period, "the Warring States period", was a time of civil wars and social upheavals that took place almost continuously in the 15th and 16th centuries. Historians even argue about its dates: most start it with the Kyōtoku incident (1454), the Ōnin War (1467) or the Meiō incident (1493), and they place its end anywhere from 1568 to 1638.

The spark came from the shogun's own family. Shogun Ashikaga Yoshimasa had no sons, so he chose his younger brother as heir. Then his wife gave birth to a son, and the lords split over who should be the next shogun. Some scholars suggest it might not have led to war if the shogun had been a better leader. In 1467 it became the Ōnin War, which devastated two-thirds of Kyoto and badly damaged the authority of the Ashikaga shoguns over the regions.

After that, the lords went home to their domains, and the real change began. Some found that the deputies they had left in charge had seized control or declared independence. This became a pattern called gekokujō, "low conquers high": capable underlings forcefully overthrew their aristocratic masters. Hōjō Sōun, a man of relatively humble origins, seized power in Izu Province in 1493. Even farmers, craftsmen and merchants rose up in revolts called ikki, and one uprising led by monks kept Kaga Province independent for nearly 100 years.

The shogun still existed, but the country belonged to warlords. It would take three men, whom modern Japan calls the three great unifiers, to bring it together again.