Abstract In stark contrast with efforts to understand the Korean War as a military and an international conflict, very little has been discussed about the complexities of Communist activism in southern Korea under the leadership of Pak Hŏnyŏng and the South Korean Workers' Party (SKWP), the largest Communist organization in southern Korea before the war. By examining the Official Documents of the SKWP, south and north Korean government documents, and American military government reports, I will argue that analyzing the 1945-1947 moment from the SKWP's perspective offers an important ideational origin to the Korean War—the elimination of "Unitary Socialism." I will trace the early history of the SKWP, from its revival as the Chosǒn Communist Party in September 1945 to the CCP's full transformation as the SKWP—a singularly representative party of the southern Left in November 1946. I will argue that the SKWP's adoption of a rigid and an exclusionary Communism and support for north Korea assured the death of Yǒ Un-hyǒng's "Unitary Socialism" and any possibility for a peaceful non-ideological and nationalist unification of Korea.The SKWP launched Pak's personality cult, supported north Korea's land reforms, survived a minting scandal, and launched two major small civil wars against the Rightists to realize Communist political supremacy in southern Korea. In response, the southern Rightists used the Korean national police and the extremely anti-Communist Northwest Youth Corps, and established a Legislative Assembly to launch anti-Communist operations against challengers or skeptics of anti-Communism. The Korean War was a southern civil war because the SKWP's quest for a "more perfect Communist revolution" directly clashed with Right-wing attempts to impose martial law and political terrorism to consolidate an anti-Communistic security state. The clash lit the fuse of the war by expulsing "Unitary Socialism" and introducing a Manichean Cold War political culture in the Korean peninsula. The war was north Korea's extension of the struggle between the southern Left and Right for political supremacy in southern Korea.