The USA saw the invasion as a test of containment and the UN as a threat to peace, so they defended the South, then pushed north.
North Korean forces invaded on 25 June 1950 and quickly pushed the South's army back. US President Harry Truman feared that if the invasion succeeded, communists would attempt more advances, and other countries would doubt American promises. This was containment: resisting the spread of communist power. He sent air and naval support, then ground troops. Defending Rhee's government did not mean it was fully democratic, or that the USA approved of everything it did.
The United Nations (UN), an organisation of states that aims to keep the peace, condemned the invasion through its Security Council. It recommended that members help South Korea. This was collective security: countries acting together against a threat to peace.
Normally, the USSR could block such a decision with its veto. But it was boycotting the Council, refusing to attend, because China's seat was still held by the Nationalist government in Taiwan, not Mao's communist government. So Soviet absence meant no veto, not Soviet agreement. Troops from several countries fought under a US-led UN command.
South Korean and UN forces held on around Pusan, a port in the south-east. In September 1950, UN forces landed at Inchon, near Seoul, behind the North's army. This threatened the North's supply routes and turned the war around.
Next, UN forces crossed the 38th parallel into North Korea. This went beyond saving the South: it aimed to reunite Korea under a non-communist government. That threatened China's communist neighbour and brought hostile troops close to China.
- Containment
- Stopping communism from spreading.
- Soviet boycott
- USSR absent, so it could not veto UN action.
- Inchon, September 1950
- UN landing that turned the war.
Worked example: Why Truman thought Korea mattered
Source A calls the invasion armed communist expansion and links the response to international rules.
- Detail: Truman calls it more than a Korean quarrel.
- Link: if it succeeded, he feared more attacks and loss of allies' trust.
- Inference: defending the South was about containment worldwide.
Watch out for this
Because the Security Council approved action, the USSR must have supported it.
The USSR was boycotting the Council. Its absence meant no veto, not agreement.
Check your understanding
Why could the UN Security Council act against North Korea?
- The USSR was boycotting the Council, so it did not use its veto.
- The USSR voted in favour of helping South Korea.
- China used its veto to support the South.