If we are talking about President Franklin D. Roosevelt, he clearly followed an interventionist policy after the beginning of World War II. Many people in the United States continued to believe in isolationism, but Roosevelt did not agree with them, particularly after the war started in September of 1939.
President Roosevelt strongly believed that the United States needed to get involved in world affairs. He felt that Nazi Germany was a menace to democracies everywhere and to world peace in general. Once WWII started, he was more able to convince other people to come around to his view of Germany. He began to act in a very interventionist way, helping the Allies through such programs as Lend-Lease and the Destroyers for Bases deal. Once WWII started, Roosevelt definitely pursued an interventionist policy.