The correct answers are B) President Truman asked the UN to help. D) The UN wanted to protect a democratic nation.
The UN got involved in the Korean War because "President Truman asked the UN to help," and "The UN wanted to protect a democratic nation.
We are talking about the tense years of the Cold War. The Soviet Union was trying to spread Communism in different parts of the world, Korea included. Indeed, the Soviet Union had been supported North Korea.
That is why the United States and the United Nations had to intervene to help South Korea. The US federal government feared the "Domino Effect," in which, if one nation converted to Communism, it was probably that other nations would follow.
During the Cold War, the Soviet Union and the United States also competed in the arms race and the space race.