The USA saw the invasion as a test of containment, and the UN treated it as a threat to international peace. Their intervention stopped the South's defeat, but advancing into the North created new risks.
North Korean forces invaded on 25 June 1950 and rapidly pushed the South's army back. US President Harry Truman feared that a successful invasion would encourage further communist advances and make other countries doubt American support. This was containment: resisting the further spread of communist power. He ordered air and naval support, followed by ground troops. Supporting Rhee against invasion did not make Rhee's government fully democratic or mean that Washington approved everything it did.
The United Nations (UN), an organisation of states seeking to maintain peace, condemned the invasion through its Security Council and recommended help for South Korea. This was collective security: countries acting together against a threat to peace. The USSR could normally block such a resolution with its veto. However, it was boycotting the Council, refusing to attend because China's seat was still held by the rival Nationalist government based in Taiwan rather than Mao's PRC. Soviet absence meant no Soviet veto, not Soviet agreement. Troops from several countries served under an American-led UN command.
South Korean and UN forces held a defensive area around Pusan, a port in Korea's south-east. In September, UN forces landed at Inchon near Seoul, behind the North's advance. Bringing troops ashore there threatened the North's supply routes and helped reverse its advance. The next move, across the 38th parallel into North Korea, went beyond saving the South: it attempted reunification under a non-communist government. That threatened China's neighbouring ally and brought hostile troops closer to China.
Worked example: Infer why Truman thought Korea mattered
Source A describes the invasion as armed communist expansion and links the response to international rules. What can you infer from those details?
- Truman described more than a dispute between two Korean governments: he presented the invasion as a wider challenge to peace.
- If such an attack succeeded, he feared other advances would follow and countries relying on American or UN support would lose confidence.
- The inference is that he saw defending the South as protecting containment and international security beyond Korea. The speech establishes his argument; it does not prove that every communist action followed one plan.
Watch out for this
Because the Security Council approved action, the Soviet Union must have supported intervention.
The Soviet representative was absent in a boycott. Its unused veto allowed the resolutions to pass; absence was not an approving vote.
Check your understanding
Why did Soviet opposition fail to block the Security Council's response?
- The USSR gave up its veto when Chinese communists won power in 1949.
- The USSR attended but accepted intervention because it expected the action to remain limited.
- Its representative was absent, so it did not use the veto that could have blocked the resolutions.