The US didn't rationalize, the UN rationalized.
The US government felt that the UN was right and stepped up as a member of the security council to protect the Korean people being slaughtered by the sneak attack from USSR weapons and North Korea.
The US was part of a United Nations force fighting North Korea and its allies.
The US Air Force did not exist in either WWI or WWII. The US Air Force was created in 1947 and fought its FIRST war in Korea in 1950.
1945
The United Nations approved the use of military force against North Korea.
Almost two weeks
Korea
The UN security council voted to help South Korea resist the North Korean invasion. The US was one of the members of that council. It also had the largest army, navy and air force.
South Korea
No. The US set up the government of South Korea using former Korean administrators from the Japanese Colony of Korea. The Soviet Union set up the government of North Korea using by supporting Korean Communist Rebels that had lived in the Soviet Union and Manchuria during the Japanese Colonial Period.
North Korea has issues with the US. But the US does not hate Korea. The US has a strong and very old alliance with China that makes then stronger than Korea.
South Korea
No. The US leads the world in enforcing sanctions against North Korea.