What motivated U.S. foreign policy from 1930-1975?
There is not merely one answer to this question, as US foreign policy in the time period was not consistent, and it spans several Presidents, each with their own beliefs and agenda for America's role in the world.
During the Great Depression in the 1930s, Americans did not want anything to do with other countries' issues. They were focused on economic and personal survival, and had no appetite for foreign adventures. The politicians knew this and catered to the isolationist sentiment of the time.
This ended immediately with the attack on Pearl Harbor. Since that time we have been an interventionist country, playing a crucial role in the Allied victory in World War II.
After Word War II, the main motivation for our foreign policy was the containment of communism. This prompted foreign aid on a massive scale, some of which was military aid. We formed a long term alliance and military presence in Europe and fought proxy wars against communism all over the globe.
Increasingly, at the end of your time period, US foreign policy became about securing oil resources for our growing economy.
This is a VERY broad question, I'll give a very basic answer.
1930s -- isolationism and a desire to avoid getting dragged in to another war the way we were thought to have been dragged in to World War I.
Early 1940s -- World War II
Late 1940s -- beginning of the Cold War. During this time the US was trying to figure out what its policy toward the Soviet Union and communism would be.
From then on, the motive behind US foreign policy was to contain communism. It was that which drove the US to become involved in the Korean War and the Vietnam War.