Historians have settled on the basic cause of the Korean War: the decision by North Korean leader Kim Il-sung, approved by Stalin in March 1950 and tacitly backed by Mao, to unify the peninsula by force, exploiting a window in which the United States had reduced its military presence in the Pacific. The gamble misfired at 4 a.m. on 25 June 1950, when the Korean People’s Army crossed the 38th parallel, triggering a three-year war that would draw in the United States, China, and the rest of the United Nations.
The war did not come out of nowhere. By June 1950, the two Korean states had been on a war footing for two years, Korean frontier had been the site of constant skirmishes. Skirmishes, however, are not wars. The decision that made the war was Kim Il-sung’s, made possible by Stalin’s, and it has to be explained at three levels: the Korean, the Asian, and the global.
The Korean level
Kim Il-sung wanted a unified communist Korea. He had spent the Japanese occupation fighting as a guerrilla in Manchuria and had risen in the Soviet-controlled Korean army that entered the north in August 1945. By 1950 he had consolidated power, eliminated domestic rivals, and built a conventional army larger and better equipped than the southern one. The army included 150 Soviet-built T-34 tanks, more than 200 aircraft, and a logistics chain modeled on the Red Army.
The North Korean government also had a genuine claim to legitimacy in the south. The Republic of Korea in the south was unpopular in many provinces, had used extreme violence to suppress the Jeju uprising of 1948 to 1949 and the Yeosu–Suncheon rebellion of 1948, and was led by Syngman Rhee, a 75-year-old conservative who had been out of the country for thirty-three years. Kim calculated that the southern population would welcome the northern army as liberators. He was not entirely wrong; the speed of the initial advance suggested real indifference, and in some areas active support, for the southern government.
The Asian level
The communist victory in China in 1949 changed the strategic geography. Until then, North Korea had been an isolated communist outpost on a peninsula dominated by American sea power. After October 1949, it had a continental ally. Mao had committed 300,000 Korean War veterans of the Chinese People’s Liberation Army to the new People’s Republic, and by early 1950 the PVA had the capacity to intervene in Korea if needed.
Stalin’s calculus shifted with the Chinese victory. The Soviet ambassador in Pyongyang, Terentii Shtykov, cabled Moscow in early 1950 with Kim’s plan for a southern offensive, and Stalin replied cautiously, advising against the operation, on January 30, 1950. By March 1950 Stalin had changed his mind. The decisive factor, according to the most recent archival work, was the January 1950 collapse of the U.S.–Soviet talks on the future of Germany and the December 1949 Soviet test of an atomic bomb, both of which suggested to Stalin that the United States would be reluctant to escalate in Asia. Stalin approved the operation on the condition that the Chinese provide backup if American forces intervened.
The American level
The American position in early 1950 made the gamble plausible. Secretary of State Dean Acheson had given his famous National Press Club speech on January 12, 1950, defining the American “defense perimeter” in the Pacific as running from the Aleutians through Japan, the Ryukyus, and the Philippines. Korea was explicitly outside that perimeter. The speech was meant to clarify American priorities, but it also told Moscow and Pyongyang that an invasion of the south would not trigger American ground forces.
The American military presence in East Asia was small. U.S. forces in Japan totaled roughly 80,000, most of them occupation troops scheduled to be withdrawn. The State Department had not consulted the Pentagon before Acheson’s speech, and the Joint Chiefs were alarmed by the implications. When the war started, the United States had no plans for a Korean campaign, no significant stockpiles of equipment earmarked for the peninsula, and a doctrine, NSC-68 (which was not yet adopted), that did not formally extend to Korea.
The decision
Kim Il-sung’s request for Soviet approval had been on Stalin’s desk since at least September 1949. Stalin had been unresponsive, in part because he distrusted Kim and in part because he was preoccupied with the Soviet atomic bomb project. In March 1950, after a personal visit by Kim to Moscow, Stalin approved. The cable went through Soviet channels on May 14, 1950, six weeks before the invasion. Stalin’s conditions were that the operation must be swift, that Kim should not be drawn into a long war, and that the Soviet Union would not be publicly involved. Mao gave his own approval in May 1950, committing the PVA to intervene if American forces crossed the 38th parallel.
On June 25, 1950, the Korean People’s Army crossed the 38th parallel at 4 a.m. The war began.
For the broader arc of the conflict, see the Korean War overview. For the diplomatic consequences of the communist victory in China that made the war possible, see the Sino-Soviet split.