After World War II, Kishi was imprisoned for three years as a suspected Class A war criminal. However, the U.S. government did not charge, try, or convict him, and eventually released him as they considered Kishi to be the best man to lead a post-war Japan in a pro-American direction. With U.S. support, he went on to consolidate the Japanese conservative camp against perceived threats from the Japan Socialist Party in the 1950s. Kishi was instrumental in the formation of the powerful Liberal Democratic Party (LDP) through a merger of smaller conservative parties in 1955, and thus is credited with being a key player in the initiation of the "1955 System", the extended period during which the LDP was the overwhelmingly dominant political party in Japan.[3][4]
As prime minister, Kishi's mishandling of the 1960 revision of the U.S.-Japan Security Treaty led to the massive 1960 Anpo protests, which were the largest protests in Japan's modern history and which forced him to resign in disgrace.