The Sengoku Period is a period of great upheaval in Japanese history. It is usually said to have lasted from 1467 (the beginning of the Onin War) to 1568 (when Oda Nobunaga entered Kyoto). During this period, the fighting in Japan was almost constant. The period, however, led to the unification of Japan.
During this period, there was a central government. However, it was rather weak. Various economic and political forces led the nobles in the provinces to seek independence and power of their own. This led to a great deal of fighting as various nobles tried to get more power. The fighting only ended when Oda managed to gain control of most of Japan, leading to the establishment of the Tokugawa Shogunate.