Kim Il Sung asked Stalin for permission to invade. Stalin said no — then changed his mind after the Soviets got the atom bomb. On June 25, 1950, the tanks rolled south.
Kim Il Sung had been lobbying Stalin to approve an invasion of South Korea since 1949. Stalin repeatedly said no, believing the timing was wrong and that American intervention was too likely. By early 1950, his calculus had changed: China had fallen to the Communists, the USSR had tested its first atomic bomb, and American forces had withdrawn from Korea. On January 30, 1950, Stalin sent his approval.
American intelligence was caught completely off guard. Despite detecting northward movements of Korean People's Army units, intelligence agencies assessed invasion as 'unlikely.' The United States had only 200–300 troops on the Korean peninsula at the time. The Central Intelligence Agency had not yet figured out how to anticipate Soviet-backed military action.
At 4:40 a.m. on June 25, 1950, North Korean artillery opened fire across the 38th parallel. Infantry and Soviet-supplied T-34 tanks poured south along multiple axes of advance. The attack was a complete surprise. South Korean forces, equipped with only small arms and lacking tanks or anti-tank weapons that could stop the T-34, collapsed rapidly.
Seoul fell on June 28 — just three days after the invasion began. South Korea's 95,000-strong army had been reduced to fewer than 22,000 effective soldiers in under a week. The capital was in enemy hands and North Korean forces were racing south. It appeared that South Korea would be overrun within weeks.
Truman's response was immediate and unilateral. He committed US forces without a congressional declaration of war, calling it a 'police action.' The decision benefited from a remarkable diplomatic accident: the Soviet Union had been boycotting the UN Security Council to protest the exclusion of Communist China, so it couldn't veto the Security Council resolution authorizing military intervention.
The UN resolution gave the conflict international legitimacy, and eventually sixteen nations contributed forces — Britain, Canada, Australia, Turkey, France, and others alongside the United States and South Korea. But the practical burden fell overwhelmingly on American troops, who made up the vast majority of UN forces and nearly all of its leadership. For American soldiers, what began as an occupation duty in Japan suddenly became active combat.
The speed of the North Korean advance shocked both South Korea and the United States. American forces rushed from Japan were thrown into combat with minimal preparation, inadequate equipment, and no recent combat experience. The first American unit to engage the enemy — Task Force Smith — was routed within hours. Korea was not the easy containment mission anyone had imagined.
In the United States, the war began with strong public support but without the sense of national purpose that had defined World War II. There was no declaration of war, no full mobilization, no shared sacrifice. American soldiers fought and died in a conflict that most Americans struggled to find on a map — a pattern that would define the Cold War era of limited, undeclared wars.