1) In the 1920s, the US pursued an isolationist foreign policy in the aftermath of World War 1 and refused to join international agreements or recognize the Soviet Union.
2) During World War 2, the US initially pursued neutrality but gradually increased aid to Britain through cash-and-carry and eventually lend-lease programs before entering the war after the Japanese attack on Pearl Harbor.
3) The US mobilized its massive industrial capacity to support the war effort as an "arsenal of democracy" and emerged from the war as the most powerful nation with unprecedented economic prosperity and an expanded role for the federal government.