The term "Isolationism" refers to the direction of US foreign policy in the wake of World War I. It is based on the idea of non-interference in armed conflicts outside the American continent. In many respects, the policy of isolationism began with the decision of the United States Senate not to join the League of Nations in 1919.
The breakdown of the international order in the 1930s, thanks to the Japanese invasion of Manchuria and China, the Italian invasion of Abyssinia and the rise of Adolf Hitler, all contributed to the strong support isolationism had among the public and the Congress. The worsening international situation and the growing threat of a new world war led Congress to pass a series of Neutrality Acts beginning in 1935. A two-Year Amendment to Neutrality Law adopted in 1937 lifted the arms embargo and allowed the US to sell military goods to belligerent nations on a cash and carry basis. Two years later it was renewed again.
The rise in isolationism in the interwar years is explained by the expansion of its social base (according to surveys, at the time of the outbreak of the war, 71% of the population were against America’s participation in the war). Among the most famous isolationist groups were Mothers’ Movement (1939) and America First Committee (1940-1941).
Realizing the need to help the Great Britain and the Soviet Union in the fight against Nazism, Roosevelt initiated a shift in US foreign policy towards internationalism. He began this effort with his December 1940, Arsenal of Democracy speech, followed by his January 6, 1941, Four Freedoms address. Three months later on March 11, 1941, FDR signed the Lend Lease Act, which was ultimately followed by the signing of the Lend Lease Consideration Agreement a year later.
The turning point in the fight against anti-war public sentiment was the tragedy of Pearl Harbor. This event convinced many Americans that they could no longer remain neutral to what was happening in Europe and Asia.