Civil war began in Kyoto in 1467 as a struggle over succession to the shogunate. Rival claimants and their followers burned down temples and mansions, destroying much of the city and its treasures. Once Kyoto was laid waste, war spread to outlying areas. When the shogun could no longer protect cities, merchants banded together to hire mercenaries. In the political vacuum, the Lotus League, a commoner-led religious sect united by faith in the saving power of the Lotus Sutra, set up a commoner-run government that collected taxes and settled disputes. In 1536, during eight days of fighting, the powerful Buddhist monastery Enryakuji attacked the League and its temples; burned much of the city; and killed men, women, and children thought to be believers.

In these confused and violent circumstances, power devolved to the local level, where warlords, called daimyo (DIGH-myoh), built their power bases. To raise revenues, they surveyed the land and promoted irrigation and trade. Many of the most successful daimyo were self-made men who rose from obscurity.

The violence of the period encouraged castle building. The castles were built not on mountaintops but on level plains, and they were surrounded by moats and walls made from huge stones. Inside a castle was a many-storied keep. Though relatively safe from incendiary missiles, the keeps were vulnerable to Western-style cannon, introduced in the 1570s.