The Sengoku was a period of political upheaval and warlordism in Japan, lasting from the Onin War of 1467-77 through the reunification of the country around 1598.
The Onin War was fought over a disputed succession in the shogunate; in the end, nobody won. For the next century and a half, local daimyo or warlords vied for control over the different regions of Japan.
Japan's "Three Unifiers" brought the Sengoku Era to an end. First, Oda Nobunaga conquered many other warlords, beginning the process of unification. Toyotomi Hideyoshi continued the pacification after Nobunaga was killed. Finally, Tokugawa Ieyasu established the stable Tokugawa Shogunate, which ruled until the Meiji Restoration in 1868.
"Japan's Sengoku Period is sometimes compared to Europe's Dark Ages, as a chaotic transition time between political systems."