The party was outlawed by the U.S. occupation authorities due to the party being an aggravating opposition to South Korea and the US, but the party organized a network of clandestine cells and was able to obtain a considerable following. It had around 360,000 party members.[3] In 1947, the party initiated armed guerrilla struggle. As the persecution of party intensified, large sections of the party leadership moved to Pyongyang.
The party was opposed to the formation of a South Korean state. In February–March 1948, it instigated general strikes in opposition to the plans to create a separate South Korean state.[4] On 3 April 1948, the party led a popular uprising on Jeju island, against the unilateral declaration of the foundation of South Korea. In the suppression of the revolt, thousands of islanders were killed (see Jeju massacre), largely by forces of the South Korean Government.[citation needed]
In the Korean War, 60,000 to 200,000 members of the party and suspected communist supporters, many of them civilians, were massacred by the South Korean Army with supervision of the US army[7] in what became known as the Bodo League massacre.
