Shogunate
The military leadership of Japan lead by the Shogun. It also held civil and judicial power. Originally the Emperor bestowed the hereditary title, though sources differ on who was the first Shogun.
The Shogunate turned dictatorship in the year 1600 with the Battle of Sekigahara when Tokugawa Ieyasu (in the east) defeated the Toyotomi clan (in the west). This was the beginning of over 250 years of war free history. A stratified structure to ensure order and power was created with the Shogun, samurai and Emperor at the top, and everyone else underneath it. Tokugawa also severely restricted the western lords, causing generations of frustration.
Combined with a weak stance on foreign powers, the ill-treatment of the west lead the samurai there to rebel. The Shogunate ended in 1868 with the Meiji Restoration.
Shogun Tokugawa Ieyasu by Utagawa Yoshitora
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Author's Notes
This is a super quick overview of a topic with a long complicated history. If you're interested in the subject, I encourage you to learn more than I can give in this little article. Shogo has a terrific short video on the history of the Shogunate and Japan. If you haven't checked out his videos, do it! He's Japanese and is interested in history and preserving Japanese culture. The videos are short, enjoyable, and very informative.
Shogun (Wikipedia)
Tokugawa shogunate(Wikipedia)
A few more resources to get you started