[8] Following the signing of the San Francisco Peace Treaty in 1951, the Japan Socialist Party dissolved into chaos and internal bickering between moderate reformist socialists and more radical revolutionary socialists over the issue of whether or not to support the Treaty.
As a result of the JSP split, some of its members formed a more centrist social democratic party, while others formed a more radical socialist party.
Both groups claimed the name Nihon Shakaitō (日本社会党) but different English translations, and are known as the Left Socialist Party of Japan and the Right Socialist Party of Japan, respectively.
In early 1955, the Left Socialists and the Right Socialists reconciled and merged to reform the JSP, months before the Liberal Democrat Party was created through the merger of the Liberal and Democrat parties.
The Young Socialists, a newly formed youth organisation which retains full membership in the International Union of Socialist Youth, is said to be inherited from the political tradition of the Right Socialist Party.