The destructive actions of the U.S. from the 1940s to the 1970s were justified by the need to fight totalitarianism. Reference: World War II, The Cold War and Decolonization
It could be argued that the destructive actions of the U.S. from 1940 to the 1970s were made in an attempt to fight different forms of totalitarianism. In 1941, the United States became involved in fighting with the Allies in World War II to fight the Axis powers, including the Nazis in Germany. The Axis powers all practiced a form of totalitarianism, meaning a political system in which the state has complete control.
After World War II, the U.S. engaged in a series of wars to fight communism, which was another form of totalitarianism. The series of wars to contain and then roll back communism was referred to the Cold War, as it was usually fought through a series of proxy wars rather than direct engagement with the other world superpower, the Soviet Union.
The first major action of the U.S. that could be considered destructive after World War II was the Korean War, technically fought under the aegis of the United Nations (though U.S. troops made up most of the forces that were allied with South Korea). The war was in part a result of decolonization, as the Japanese had occupied Korea during World War II, and after the war, Korea remained divided between the communist north and the nominally democratic south. The North Koreans crossed the line that separated the two nations (at the 38th parallel) in 1950, and the three-year war was in part an attempt to contain communism and fight totalitarianism. The war ended in a stalemate, and the country remains divided between a communist North and democratic South.
The Vietnam War, in which the U.S. engaged troops in 1965 (after several years of sending advisers to the South Vietnamese military), was an attempt to fight the communist forces of North Vietnam and communist sympathizers in South Vietnam. Like the Korean War, the war in Vietnam was in part an effect of decolonization, as Vietnam had been occupied by the Japanese during World War II. After the war, the French, who had been the former colonial power in that part of Indochina, tried to reassert control, but they were pushed out in 1954. The country was divided at the 17th parallel, and the war started when the North allegedly fired on a U.S. destroyer in the Gulf of Tonkin. The war, which the U.S. lost by 1975 with the fall of Saigon, was an attempt to roll back communism, a form of totalitarianism.