Answer:
Isolation
Step-by-step explanation:
WAY back in the day, President Washington warned the U.S. in his farewell address not to become entangled with foreign nations since France was meddling in their politics. This established an impromptu policy of isolationism, where the U.S. would not get involved in any foreign conflicts, or entangle themselves in the political disputes of other nations. This carried through to WWI, until the Germans repeatedly sunk civilian vessels and killed innocent Americans. They still remained isolationists post WWI with the League of Nations, not entering into it.
When WWII came around, the attack on Pearl Harbor was a direct attack on America, and they couldn't remain isolated any longer. After the war, they joined the United Nations, and did end up entangling themselves in many nations in order to stop the spread of communism.