The southern government was barely inaugurated before it had to contend with a left-wing guerrilla movement. Although this movement received support from the North, it had its greatest strength in the South, particularly in the southernmost provinces and on Jeju Island off the southern coast. The ROK Army required the better part of 1948 and 1949 to suppress the rebellion, and it did so with the support—and often the direction—of a 500-man contingent of American advisers. By early 1950 the guerrillas appeared to be defeated. Although the Soviets withdrew their troops from the Korea Peninsula at the end of 1948, the Americans, concerned about the rebellion in the South and the potential of invasion from the North, delayed their withdrawal until the end of June 1949. By this time, troops from both North and South Korea were concentrated along the 38th parallel. In May 1949 border fighting broke out and continued, on and off, through December. Thousands of troops were involved. According to formerly classified American reports, the South provoked the majority of the 1949 border fights, prompting American advisers to try to restrain the South. After a U.S. request, military observers from the United Nations were dispatched to Korea. In addition, the United States denied the ROK Army’s requests for combat airplanes and tanks. At about the same time, U.S. Secretary of State Dean Acheson delivered what became known as the “Press Club” speech in Washington, D.C., in which he was ambiguous about whether the United States would defend the ROK in a war.
Although Kim Il Sung would be eager to fight in 1950, he was not ready in the summer of 1949. Large contingents of his best North Korean soldiers were still in China, fighting on the side of the Communists in that country’s civil war. In the early months of 1950, however, tens of thousands of these soldiers returned to the DPRK, including the 6th Division under General Pang Ho-san, which had a distinguished record in China. In May 1950 Kim perched this division just above the 38th parallel. He hoped that the summer of 1950, like the summer of 1949, would bring South Korean provocations, which he could use to justify an invasion by the North. Kim claimed he got his provocation with a minor lunge by the South across the parallel in the early morning hours of June 25, 1950. Whether or not the South lunged across the parallel still awaits further evidence, but the North bears the major responsibility for escalating a minor skirmish to the level of massive conventional warfare.