In 1960, as Speaker of the House of Representatives, he presided over the ramming through the Diet of the U.S.-Japan Security Treaty that cemented in place the U.S.-Japan alliance and allows the United States to maintain military bases on Japanese soil.

In the late 1940s, he became internationally famous as the lead defense attorney former prime minister Hideki Tōjō during the Tokyo War Crimes Trials.

However, rather than return to his liberal ways of the 1920s, he retained his image as an arch-conservative by vocally advocating revision of Japan's postwar constitution to restore aspects of the prewar system.

[5] Despite his supposedly neutral status, Kiyose cooperated with conservative prime minister Nobusuke Kishi in calling for a surprise snap vote on the Treaty on May 19, 1960, in what became known as the "May 19 Incident."

[6] He then struggled his way to the rostrum amidst the scrum with the assistance of the police officers and gaveled the vote through with only members of the conservative Liberal Democratic Party present.