Abstract: From the perspective of the interaction between transnational structure and political party initiative, the pre-war(1920s—1940s)relationship between the Japanese Communist Party and the social democratic parties had five characteristics. First, the establishment of the JCP in 1922 preceded and stimulated the emergence of social democratic parties, deviating from the European model where communist parties split from left wings of social democratic parties. Second, the high pressure of the imperial system, the strategic discipline and localization pressure of the Comintern, and the “triple squeeze” of the militaristic fascist system have jointly shaped the narrow survival space and strategic choices of political parties. Third, the relationship between the Communist Party of Japan and social democratic parties presented a dynamic game of “anti-communism and united”, with fierce competition between the two sides in terms of ideology, organizational resources, and united front strategies, resulting in repeated defeats of the concept of a unified proletarian political party. Fourth, the brutal suppression by the Japanese government not only dealt a heavy blow to the Japanese Communist Party, but also forced social democratic parties(especially the right-wing)to gradually abandon socialist principles under fascist pressure, turn to “national socialism” and ultimately be engulfed by the New Order Movement, profoundly revealing the fragility of the reformist line under the rise of militarism. Fifth, the genealogy of the Communist Party of Japan and the three major social democratic parties, the pre-war mode of opposition, and core differences deeply influenced the complex relationship between the Communist Party and the Socialist Party after the war, which was both united and opposed.

Key words: modern Japan, socialist political parties, Japanese Communist Party, social democratic parties