North Korea Invades, June 1950
25 June 1950: A Sunday at Dawn
At around 4am on a wet Sunday morning, around 90,000 North Korean troops, supported by 150 Soviet T-34 tanks, swept across the 38th parallel. Within three days the southern capital, Seoul, had fallen. Most of the South Korean army was overwhelmed or scattered. Kim Il Sung had assumed Washington would not fight, that the south would rise to welcome him, and that the war would be over in weeks. Two of those three assumptions were wrong almost immediately.
North Korea invaded South Korea on 25 June 1950 with around 90,000 troops and 150 Soviet T-34 tanks. Seoul fell within three days. The South Korean army (ROK) was largely shattered.
Why the North Invaded
Kim’s decision to attack was the product of three reinforcing factors. First, Korean nationalism: both Pyongyang and Seoul claimed to rule the whole peninsula, and Kim believed force was the only route to reunification. Second, military superiority: Stalin had supplied tanks, planes and 135,000 trained troops while Washington had deliberately denied Rhee heavy weapons. Third, signals from the United States. In January 1950 Secretary of State Dean Acheson had publicly placed Korea outside America’s defence perimeter in Asia. After persistent lobbying through 1949–50, Stalin finally gave Kim permission to invade in April 1950, on condition that Mao agreed too — which he did in May. Kim assumed a southern uprising would topple Rhee within weeks; Soviet planners gave the campaign a one-month timetable. The first part of the gamble worked. The second did not.
Truman’s Response: From Containment to Action
News of the invasion reached Truman that evening. He was clear: this was the first armed test of containment on the Asian mainland. To do nothing would be to encourage further aggression and confirm Republican accusations that he had been weak in Asia. Within two days Truman ordered US air and naval forces to defend the south. Days later he committed ground troops. Crucially, he framed the conflict not as a war but as a police action conducted under United Nations authority — meaning he never asked Congress for a formal declaration of war.
Korea itself was strategically marginal — it had no oil, little industry beyond what Japan had built, and no natural alliance value. What made it matter was symbolism. After ‘losing China’, Truman could not afford to lose Korea too. The 1950 Cold War atmosphere — Soviet atomic test in August 1949, Sino-Soviet Treaty in February 1950, Senator McCarthy’s anti-communist crusade — meant Korea was read in Washington as a probable Soviet test of Western will.
The UN Steps In — Because the USSR Was Absent
On 25 June the United States took the case to the UN Security Council. The Council passed Resolution 82, condemning the invasion and demanding North Korean withdrawal. Two days later Resolution 83 authorised member states to use force to repel the attack. The USSR could have vetoed both resolutions and stopped the UN intervention dead. It did not — because the Soviet ambassador was not in the room.
Since January 1950 the USSR had been boycotting the Security Council in protest at the UN’s refusal to give China’s seat to Mao’s communist government rather than the defeated Nationalists on Taiwan. Stalin gambled that the boycott would force a change. Instead, it removed the Soviet veto at the very moment Washington needed UN cover. Stalin reportedly fumed at the news but did not return until August. By then the war was already a UN war in name.
The USSR was boycotting the UN Security Council in June 1950 in protest at the China seat going to Taiwan rather than Mao’s communist regime. Without a Soviet veto, Resolutions 82 and 83 passed, authorising force to defend South Korea.
The Pusan Perimeter
Even with US troops arriving from Japan, the early weeks were a disaster. South Korean and US forces were pushed steadily down the peninsula. The first US units committed — under-strength, under-equipped occupation troops sent quickly from Japan — were overrun at the Battle of Osan on 5 July 1950. By August they were defending a small rectangle in the south-east around the port of Pusan — the Pusan Perimeter. They held by a thread, helped by US air superiority, fresh divisions arriving from Japan, and the long supply lines that now stretched the over-extended North Korean army. By mid-September UN forces had stopped retreating. The next move was about to come from MacArthur, far to the rear.
Sixteen UN member states sent combat troops — including Britain, Australia, Turkey, France, Canada and the Philippines — with around 90 percent of non-Korean forces being American. General Douglas MacArthur was appointed UN Commander, but reported to Truman, not to the UN. The legal fiction of UN command was important: it provided diplomatic cover for what was otherwise a US-led war against a Soviet-supported regime.