Origin and causes
In 1910, Japan put Korea under Japanese rule and was still ruling Korea when World War II ended. When Japan surrendered, the United States and the USSR agreed to split Korea into two temporary occupation zones with USSR occupying the North and USA occupying the South. This was, at first, to be for a short time.
At the Moscow Conference of the Council of Foreign Ministers in December 1945, the United States and USSR agreed on Korea having a provisional government (a temporary government set up quickly before a real government is ready). This became difficult because of the growing Cold War.
The Cold War was an important cause in the Korean War. Relations between the two occupying powers were bad and when China became Communist in October 1949, the President of the USA, Harry Truman, was very worried that other countries around China may also become Communist, such as Japan. The American Army was about one twelfth the size of five years earlier and Joseph Stalin had recently lost a Cold War dispute over the Berlin Blockade and subsequent airlift.