The policy the US followed in the 1920's and 1930's was called?

Isolationism. The Fordney and McCumber tariffs of the 20's boosted American economy, and prevented foreign goods being popular because of high tax rates. However during the war FDR collaborated with British allies, under policies such as lend lease and the cash and carry system which supplied British allies with artillery, ammunition and loans. This could question the extremeness of the isolationist policy, which inevitably ended in 1941 when the USA joined the war.