During the U.S. occupation of Japan after World War II, Kishi was detained as a suspected "Class-A" war criminal from 1945 to 1948. Ultimately acquitted by the Tokyo Tribunal, he was banned from public life until 1952, when he joined the anti–Yoshida Shigeru faction and became secretary-general of the Democratic Party. Kishi was largely responsible for uniting all of Japan's conservative parties into the powerful Liberal Democratic Party (LDP) in 1955 and served as the first head of the LDP.
When ailing Prime Minister Ishibashi Tanzan resigned in 1957, Kishi assumed the post of prime minister. His first cabinet lasted only until 1958, but he quickly assembled a second cabinet that same year, which lasted until 1960.
As prime minister, Kishi authorized war reparations to Indonesia and authorized the implementation of the Japanese National Defense Plan to fortify Japanese defensive capabilities. He also traveled to the United States in 1957 to talk with President Dwight D. Eisenhower. Most notably, Kishi sought renewal of the United States–Japan Security Treaty. He again journeyed to Washington and in January 1960 returned to Japan with a new security treaty. But he was soon overwhelmed by a widespread backlash to the treaty, which many perceived as one that would sublimate Japanese interests to those of the Americans. Amid the growing public uproar, late in the evening of 19 May 1960 Kishi's government rammed the treaty through the Diet (parliament) on a snap vote. On 15 July 1960, he was forced to resign thanks in part to this heavy-handed parliamentary maneuver. Kishi died on 8 August 1987 in Shinjuku, Tokyo.
Packard, George R. Protest in Tokyo: The Security Treaty Crisis of 1960. Princeton, NJ: Greenwood, 1966.; Schaller, Michael. Altered States: The United States and Japan since the Occupation. Oxford: Oxford University Press, 1997.