How did the US policy change in the aftermath of World War II?
I assume you mean foreign policy. The major change is that the U.S. no longer was involved in a war against fascism, but found itself entangled in the Cold War, primarily against the Soviet Union.
There had been some distrust of the Soviets, even before the end of World War II; in fact there is strong speculation among historians that the Atomic Bomb was used on Hiroshima and Nagasaki primarily to end the war quickly before the Soviets became too involved in the war against Japan. After the war, the Soviets refused to withdraw their troops from East Germany and Poland, claiming that they needed a "buffer zone" against further invasions. The veracity of this claim is highly questionable. In clear violation of the Yalta agreements signed earlier, the Soviets set up puppet governments in Poland and Romania; claiming that the U.S. and its allies had effected Italy's surrender without Soviet participation. This also is highly questionable.
Thereafter, American foreign policy shifted to one of containment to prevent the further spread of Communism. It was best expressed in an article written by George F. Keenan in which he said American policy should be
a long term, patient but firm and vigilant containment of Russian expansive tendencies…. Such a policy has nothing to do with outward histrionics: with threats or blustering or superfluous gestures of outward toughness,
Later, when the Soviets backed communist insurrectionists in Greece, President Harry Truman issued a statement that became known as the "Truman Doctrine."
I believe that it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures."
Thus U.S. foreign policy after World War II became one of the containment by whatever means of the spread of communism. When one understands this policy, one readily understands the involvement of the U.S. in Korea, Viet Nam, and the Cuban Missile crisis.