In 1945, Katayama became secretary general and committee chairman of the party. Following the 1947 elections, in which the Socialist Party came in first, Katayama formed a coalition government with the Democratic Party and the Citizens' Cooperation Party. However, the influence of left-wing socialists such as Suzuki Mosaburo forced Katayama to resign early in his term.
After his resignation, Katayama became a member of the Democratic Socialist Party and advocated the maintenance of the pacifist constitution, election reform, and formation of a global commonwealth.
After his resignation, Katayama became a member of the Democratic Socialist Party and advocated the maintenance of the pacifist constitution, election reform, and formation of a global commonwealth.
The party became the largest political party in the general election of 1947, and a government was formed by KatayamaTetsu, forming a coalition with the Democratic Party of Japan, Occupation and another minor party.
However, due to the rebellion of Marxist tendencies in the party, the Katayama government collapsed.
As a result, the party was split into the Rightist Socialist Party of Japan, formed of socialists more to the center, while the Leftist Socialist Party of Japan was formed by hardline left-wingers and Marxist-socialists.