Korea is right next to Japan and had been under their control during WWII. After the war, Korea was divided into two independent countries by the US and the Soviet Union. The dividing line was set at the 38th parallel of latitude. In North Korea, the Soviets installed a communist government and armed its military. The US gave smaller amounts of aid to the noncommunist South Korea.
In June of 1949, as the communists take over China, American troops leave South Korea. North Korea begins a massive military buildup. On June 25, 1950, North Korean forces attacked across the 38th parallel. 90,000 North Korean troops, armed by the Soviets, captured Seoul, the South Korean capitol.
President Truman, remembering how appeasement hadn’t worked during WWII, announced that the US would aid South Korea. The UN agreed and also offered assistance. Remember, the USSR was part of the UN Security Council and would have had veto power, but it had been boycotting Security Council sessions because the UN had refused to seat the People’s Republic of China.
The US Constitution says that only Congress can declare war, but Truman does not wait for that. Supported by the UN resolution, Truman ordered American troops in Japan to move to South Korea. These troops had not been trained to march and fight in monsoon rains or rice paddies and they were not as well equipped as the Soviet supported North Koreans. They too were forced to retreat. Soon, fresh supplies and troops (both UN and American) came to help.
By September 1950, The UN forces were ready to counterattack. Led by General Douglas MacArthur, the noncommunist forces had a bold plan of attack to regain South Korea. MacArthur figured that because the North Korean troops had advanced so quickly, they would have had limited supply lines (route over which supplies can be delivered). He decided to attack this weakness by attacking the port city of Inchon, South Korea, which was now in enemy territory. Inchon was a poor area to land because of its swift current and treacherous tides, which also meant that it was the last place the North Koreans would expect an attack.
MacArthur was right. On September 15, 1950, the US Marines landed at Inchon and attacked the rear of the North Koreans. Communist forces begin fleeing back North, and by October, the North Koreans had been driven back north of the 38th parallel.
Now the US had to decide what to do. Communist North Korea had been sent back to their territory so they could say the UN resolution had worked and they had won the war. Or, they could send their forces North of the 38th parallel and punish the communists for their aggression. President Truman was concerned about how China would respond if the US forces invaded North Korea. China had already warned the Americans to not get close to their border. General MacArthur assures President Truman that there is nothing to worry about and that the Chinese would not get involved in the war. Because of MacArthur’s words, the US convinces the UN to take over North Korea and create a “unified, independent and democratic” Korea.
MacArthur attacks north of the 38th parallel. The terrain was mountainous and the temperatures were freezing, but still, by Thanksgiving, the US and UN forces reached the Chinese border. The Chinese sent approximately 300,000 Chinese soldiers to attack the South Korean and US forces. MacArthur had been wrong. China was now involved in the war.
Truman did not want to commit a huge number of troops to Asia, nor did he want to use atomic weapons. Truman wanted to avoid World War III. His policy was to fight a limited war (war fought to achieve only specific goals). MacArthur, on the other hand, was a soldier and wanted a complete, total victory. He didn’t agree with Truman’s policy and could not get the President to change his mind. What MacArthur did to get his way was send a letter to the House of Representatives that attacked President Truman’s policies. The President is the Commander-in-Chief of the military and a member of the military, regardless of the fact that he’s a General and a war hero, is not allowed to criticize the President. President Truman proceeded to fire MacArthur for insubordination. The American public, because MacArthur was such a hero, was upset and MacArthur returned home a hero.