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The Korean War (1950–1953) was the first major proxy war of the Cold War. It brought the USA, China, and the Soviet Union into a devastating conflict on the Korean peninsula and set the template for Cold War confrontations in Asia. This lesson examines the background and causes of the war.
Korea had been occupied by Japan from 1910 until Japan's surrender in August 1945. After the war, the Allied powers divided Korea along the 38th parallel — an arbitrary line of latitude.
| Zone | Controlled By | System Established |
|---|---|---|
| North Korea | Soviet Union | Communist state under Kim Il-sung |
| South Korea | United States | Capitalist state under Syngman Rhee |
Both leaders claimed to be the legitimate ruler of the entire Korean peninsula. The division was intended to be temporary, but Cold War tensions made reunification impossible.
| Person | Role |
|---|---|
| Kim Il-sung | Communist leader of North Korea; sought to reunify Korea under communist rule |
| Syngman Rhee | Authoritarian leader of South Korea; anti-communist, backed by the USA |
| Harry S. Truman | US President; committed to the policy of containment |
| Joseph Stalin | Soviet leader; initially cautious but eventually supported North Korea's invasion |
| Mao Zedong | Leader of Communist China (from 1949); supported North Korea with troops |
The Korean War must be understood within the broader context of the Cold War. By 1950, the world was clearly divided into two hostile blocs, and the USA was committed to its policy of containment — preventing the spread of communism.
| Cold War Factor | Impact on Korea |
|---|---|
| Truman Doctrine (1947) | Committed the USA to supporting countries resisting communism |
| Marshall Plan (1948) | Showed US willingness to invest heavily in containing communism |
| Berlin Blockade (1948–1949) | Demonstrated that the Cold War could produce major crises |
| NATO (1949) | Formalised the Western military alliance |
| Soviet atomic bomb (1949) | Ended US nuclear monopoly, changing the balance of power |
In October 1949, Mao Zedong's Communist Party defeated the Nationalists and established the People's Republic of China. This was a seismic event.
Key Term: Domino Theory — the belief that if one country in a region fell to communism, neighbouring countries would inevitably follow. This theory drove US foreign policy in Asia for decades.
Kim Il-sung was determined to reunify Korea under communist rule. He believed that the South Korean people would welcome liberation from Syngman Rhee's corrupt government.
In January 1950, US Secretary of State Dean Acheson outlined the American defensive perimeter in Asia. Crucially, he did not include South Korea in this line.
| Included in US Defensive Perimeter | Excluded |
|---|---|
| Japan | South Korea |
| Okinawa | Taiwan (ambiguous) |
| The Philippines |
Stalin and Kim interpreted this as a signal that the USA would not fight to defend South Korea.
Exam Tip: The Acheson speech is a key factor. Be careful how you phrase its significance — Acheson did not say the USA would abandon South Korea, but he failed to clearly include it in the US defensive perimeter. This ambiguity may have encouraged the North Korean invasion.
On 25 June 1950, approximately 75,000 North Korean troops crossed the 38th parallel and invaded South Korea. The North Korean People's Army (NKPA) was well-equipped with Soviet tanks, artillery, and aircraft.
The invasion was swift and devastating. The poorly equipped South Korean army was pushed back rapidly, and within days, the capital Seoul had fallen.
| Reason | Explanation |
|---|---|
| Containment | The Truman Doctrine committed the USA to resist the spread of communism |
| Domino theory | If South Korea fell, Japan and other Asian nations might follow |
| Credibility | If the USA failed to act, its allies worldwide would lose confidence |
| UN involvement | The invasion was condemned by the UN Security Council (the USSR was boycotting the Council and could not veto the resolution) |
| Domestic politics | Truman faced accusations of being "soft on communism"; failure to act would be politically damaging |
Exam Tip: The USSR's absence from the UN Security Council is a crucial detail. The Soviets were boycotting the Council in protest at the UN's refusal to recognise Communist China. This meant they could not veto the resolution authorising force against North Korea. Many historians consider this a major Soviet miscalculation.
| Key Point | Detail |
|---|---|
| Division of Korea | Split at the 38th parallel in 1945; North communist, South capitalist |
| Cold War context | Containment, domino theory, loss of China |
| Key trigger | Kim Il-sung's invasion with Stalin's and Mao's approval |
| Acheson Line | Ambiguous US defensive perimeter may have encouraged the invasion |
| US intervention | Driven by containment, credibility, and the UN mandate |
Exam Tip: For a "Write an account" question on the causes of the Korean War, structure your answer around long-term causes (Cold War context, division of Korea), short-term causes (communist China, Kim's ambitions, Acheson speech), and the immediate trigger (the invasion of 25 June 1950). Link each factor to show how they combined to produce the conflict.