The Sengoku Period or "warring-states" period, is a period of long civil war in the History of Japan that spans through the middle 15th to the early 17th centuries. It started in the late Muromachi period in 1467 with the Onin War (Onin no Ran 1467-1478), lasting through the entire Azuchi-Momoyama period, until final peace and order was achieved in 1615 of the Edo period.
Starting with and continuing after the Onin War, the central ruling authority of the Ashikaga or Muromachi Shogunate in the capital of Kyōto was ruined, leading to a complete breakdown in social order and civil war throughout Japan. Outside of the capital, the provincial daimyo and magistrates that relied on the shogunate for their own authority and power, found themselves isolated and vulnerable to not only external, but internal forces as well.
Many of the provincial daimyo, such as the Shimazu, Takeda, and Imagawa, having ruled their lands under the authority of not only the Ashikaga shogunate, but also under the preceding Kamakura shogunate, established their own independent domains. However, many others, like the Hosokawa, Shiba, and Toki found their lands taken over by their own subjects and retainers, like the Oda, late Hojo, and Saito Dosan, who had seized the opportunity to establish their own name and become new Sengoku daimyo in their own right. Also, peasants throughout Japan united with religious leaders and monks of the Buddhist Pure Land sect to form ikko ikki to rebel against and resist the rule of the daimyo. In some cases they succeeded in forming their own independent domains, of which the most famous ikko ikki in Kaga province lasted independently for almost 100 years.
This phenomenon of social upheaval where the retainers and subjects came to reject traditions and values of the prior establishment and forcefully overthrow their leaders to establish their own independence became known as Gekokujou. Literally, gekokujou means the bottom overcomes/conquers the top.
Struggle for Unification and Final Peace
The absence of a de facto central authority in the capital lasted until Oda Nobunaga's armies entered Kyōto in 1568, re-establishing the Muromachi Shogunate under the puppet shogun Ashikaga Yoshiaki to begin the Azuchi-Momoyama period. However, despite a renewed central authority in Kyōto and Nobunaga's attempt to unify the country, the struggle for power among warring states continued until unification and final peace was achieved long after his assassination in 1582.
After Nobunaga's assassination, Toyotomi Hideyoshi rose above his rivals to succeed his former lord. Hideyoshi first conquered Shikoku (Shikoku Heitei), then Kyushu (Kyushu Heitei) to finally unite all of Japan in 1590 by defeating the later Hojo clan of Sagami province in the conquest and siege of Odawara (Odawara Seibatsu).
However, immediately after Hideyoshi's death in 1598, his retainer Tokugawa Ieyasu sought to undermine the Toyotomi. After the Battle of Sekigahara in 1600 over Ishida Mitsunari, Ieyasu became the undisputed ruler and received the title of Seii Taishogun and established the Tokugawa or Edo shogunate in 1603. Eventually Ieyasu detroyed the Toyotomi in the Summer Siege of Osaka in 1615 to finally bring peace to Japan.
Sengoku period in modern culture
The game Shogun: Total War is a real time strategy game set in Japan during Sengoku jidai.
The anime and manga series InuYasha is set in the Sengoku period.