which event started the Korean war?
Answer:
The Northern communist guerrilla fighter/dictator Kim Il-sung invaded the relatively more democratic and capitalist South. Kim had the backing of the USSR and communist China, whereas the South were firmly backed by the USA and the West. Initially the South were left to their own devices (albeit with a lot of Western money and weapons) but when the communist North conquered most of the South and were close to annexing the entire country, the USA had to intervene.
This was because of the doctrine of containment. The idea was that if you let one country (in this case, Korea) fall to communism, more and more will as well. Nevertheless, the US intervened along with other United Nations forces and pushed back the North Koreans all the way to the Chinese border, at which point the Chinese entered the war directly and pushed the Western forces to the current border that North and South Korea have today.