As the first military action of the entire Cold War, North Korea invaded South Korea. This was seen by the Americans as a threat, and by July they had entered the war on South Korea's behalf to stop the spread of communism.
February, 1951: Peace Talks Begin
This isn't going anywhere. Maybe we should stop...
The US Marines launched an amphibious attack on the North Korean military, effectively breaking their supply line and recapturing Seoul, the capital of South Korea.
April 1951: MacArthur Fired for wanting to use A-Bomb
Inform General MacArthur that he is relieved of duty, effective immediately.
To prevent a quick Allied victory, the communist Chinese counterattacked, pushing both armies out of North Korea and protecting its communist ties.
July 7, 1953: Truce Signed
The UN and North Korea began to discuss peace, often bringing up the POWs. In August, the first round of peace talks fell through and fighting continued.
General Douglas MacArthur planned for the military to use the Atomic bomb, despite the war with Korea being a limited one. As a result, President Truman relieved him of duties and replaced him with General Matthew Ridgeway.
Right away, Mr. President.
Now under a new President, the US began to hint at dropping nuclear bombs on North Korean forces. Shortly after, both sides sign an Armistice.