There was no single US foreign policy from 1930 to 1952. In fact, there were really three distinct phases. From 1930 to 1940, the United States, with few exceptions, pursued a policy of isolationism. American political leaders, including, until late in the decade, Franklin Roosevelt, advocated staying out of European, and really the world's affairs. One example was the Manchurian crisis in 1931, when the US refused to impose economic sanctions on the Japanese, severely weakening the hand of the League of Nations in dealing with the attacks.
With the Japanese attack on Pearl Harbor, the United States entered the war, and what we will call the second phase of foreign policy, which was simply to ally with Great Britain and the Soviet Union to destroy the Axis Powers.
After the war, of course, the alliance with the Soviets broke down, and the Cold War commenced. The American foreign policy in the early years of the conflict was highly interventionist. Indeed, the United States was one of the world's two superpowers, and devoted its resources toward a policy of containment, or using financial aid and the threat of military force to check communist expansion. This policy resulted in US participation in the conflict on the Korean peninsula.