Historians have extensively documented the defections of communists and other members of the Japanese political left to imperialist and nationalist causes from the mid-1930s through the Second World War. Previous scholarship has focused on the role of the Kōza-ha and Nōro-ha factions of Japanese communists at the turn of the 1930s within the overarching intellectual history of modern Japan, or on aspects of leftist thought at the time with clear connections to later turns to imperialism. Research has highlighted fringe theories and aspects of mainline leftist politics that appear to foreshadow the rightward turn. However, emphasis on the flaws in the Japanese left constructs a “slippery slope” narrative of inevitable defection and obscures other aspects of their ideology. This paper addresses the extent to which this ideological and political turn can be explained within the two factions of Japanese communists through analyzing their approaches to imperialism and nationalism. Drawing on Japanese language communist periodicals and party newspapers, analysis reveals strong anti-imperialist rhetoric and Leninist theoretical complexity in both major factions that resist the contemporary rise in nationalism and imperialist apologia in leftist circles. This article argues that the turn to imperialism cannot be easily explained through theoretical failings or inevitable subsumption into Japanese nationalism in the pre-WWII era, and that further research into the rhetoric and theories of the Japanese pre-war Left can complicate the “slippery slope” narrative.