In the aftermath of Japan's defeat in World War II, under his communist elder brother's influence, Pak joined a communist group, the South Korean Workers' Party, in the American occupation zone, which later became South Korea. Pak was involved in a rebellion in Yeosu and Suncheon, Jeollanam-do, led by units of the new American-supported army. Pak was arrested and sentenced to death, but released soon after revealing the names of communist participants to the South Korean authorities. He was then released and left the army in dishonor. However; the outbreak of the Korean War enabled him to be reinstated, and he served the new country fighting against the communists.