Why did the US and UN send troops when North Korea attacked South Korea?

to create an alliance
to protect colonial interests
to follow the containment policy
to create a demilitarized zone

to follow the containment policy

Explanation:

Returning to the prewar situation and relations in the light of new circumstances was not possible for US foreign policy makers. A new model of American foreign policy leadership began to emerge and develop from the beginning of 1947. It was marked by four innovations: the Truman Doctrine, the Containment Policy, the Marshall Plan, and the system of military-political alliances. An important prerequisite for the success of the new model of US foreign policy was, for the first time, the expressed willingness of the United States to take on permanent commitments abroad The Western Hemispheres and the kind of globalization of their foreign policy, contrary to the traditional approach. Although Truman's approach was called a containment strategy, it should be emphasized that its formal formulation and verification never came to pass. The strategy of containment, understood as a term without a clearly defined profile and with ascribed meanings, emerged as an instinctive response to the perceived threat of Soviet expansionism. It was quickly observed that a thoughtful strategy to contain communist expansion would not be successful  should Western European states remain economically devastated and politically unstable. So  a program of economic self-help and US aid was developed, which, in contrast to the past patterns of behavior on the international scene and its own traditional principles of foreign policy, went beyond the usual boundaries of the Western Hemisphere. Military commitments have been taken wherever communist-inspired aggression or subversion has been estimated.

Who it was like back then