What caused partisan warfare in South Korea before the Korean War?

Partisan warfare in South Korea prior to the Korean War was primarily driven by the intense political polarization following the end of World War II and the subsequent division of the Korean Peninsula at the 38th parallel.1

The conflict intensified in 1948 when the United Nations moved to establish an independent South Korean state, a policy strongly opposed by southern communists. This opposition manifested in widespread partisan uprisings across every province south of the 38th parallel. Key factors contributing to this violence included:1

• Ideological Conflict: The struggle between competing visions for Korea—represented by Marxist revolutionaries like Kim Il-sung in the North and anticommunist leaders like Syngman Rhee in the South—created deep societal fractures.1
• Military and Civil Unrest: Events such as the Yeosu-Suncheon Rebellion highlighted the internal resistance within the South Korean military and civilian population against the Rhee government and U.S. influence.2
• Cross-Border Incursions: The North Korean regime actively supported southern guerrillas to destabilize the region and divert South Korean security forces.1

This period was marked by significant brutality, with thousands of deaths among both security forces and civilians labeled as "rightists" or "reds," creating a climate of violence that persisted until the outbreak of the Korean War.12