The Sengoku period (戦国時代, Sengoku jidai^?) or Warring States period in Japanese history was a time of social upheaval, political intrigue, and nearly constant military conflict that lasted roughly from the middle of the 15th century to the beginning of the 17th century. The name "Sengoku" was adopted by Japanese historians in reference to the Warring States period in Chinese history (Sengoku jidai (戦国時代^?) in Japanese) which preceded the unification of China. Likewise, the Sengoku period in Japan would eventually lead to the unification of political power under the Tokugawa shogunate.

Although the Ashikaga shogunate had retained the structure of the Kamakura bakufu and instituted a warrior government based on the same social economic rights and obligations established by the Hōjō with the Jōei Code in 1232, it failed to win the loyalty of many daimyo, especially those whose domains were far from Kyoto. As trade with China grew, the economy developed, and the use of money became widespread as markets and commercial cities appeared. This, combined with developments in agriculture and small-scale trading, led to the desire for greater local autonomy throughout all levels of the social hierarchy. As early as the beginning of the 15th century, the suffering caused by earthquakes and famines often served to trigger armed uprisings by farmers weary of debt and taxes.

The Emperor of Japan is "the symbol of the state and of the unity of the people" according to the 1947 Constitution of Japan, which dissolved the Empire of Japan when it was adopted by the Postwar Japanese government. He is a ceremonial figurehead under a form of constitutional monarchy and is head of the Japanese Imperial Family with functions as head of state. He is also the highest authority of the Shinto religion as he and his family are said to be direct descendants of Amaterasu.

The Emperor is called the Tennō (天皇) in Japanese, literally meaning "heavenly sovereign". He is also referred to in English as the Mikado (帝) of Japan. Currently the Emperor of Japan is the only remaining monarch in the world reigning under the title of emperor.

The Imperial House of Japan is the oldest continuing hereditary monarchy in the world. In Nihon Shoki, a book of Japanese history finished in the eighth century, it is said that the Empire of Japan was founded in 660 BC by Emperor Jimmu. The current emperor is His Imperial Majesty the Emperor Akihito, who has been on the Chrysanthemum Throne since his father the Showa Emperor (Hirohito) died in 1989.