At the end of WWII in '45, Korea was freed from Japanese control. North Korea was occupied by the Soviets while the South was occupied by American forces.
The UN troops, composed mainly of Americans, landed in South Korea in early July, but were soon pushed back on defense by the North Korean forces, forming a perimeter around Pusan to defend the line until reinforcements arrived in August
The North Korean troops were pushed back on the retreat over the 38th Parallel, and soon Seoul was recaptured along with the whole of South Korea by the end of the month
General MacArthur wanted to use the atom bomb on China, and was dismissed for insubordination by President Truman, who went back to a policy of containment