Between 1939 and 1945 the Federal Bureau of Investigation, headed by J. Edgar Hoover, monitored the political activities of President Franklin Roosevelt's anti- interventionist foreign policy critics. Hoover, whose position as FBI director was tenuous within the left-of-center Roosevelt administration, catered to the president's political and policy interests to preserve his position and to expand FBI authority. In his pragmatic effort to service administration political goals, Hoover employed illegal wiretaps, informers, collected derogatory information, conducted investigations that had the potential to discredit the anti -interventionists, forwarded political intelligence to administration officials, and coordinated some activity with British intelligence. This all occurred within a crisis atmosphere created with the onset of the Second World War, and it was this political dynamic that permitted Hoover to successfully cultivate his relationship with President Roosevelt. In the process, the administration's otherwise legitimate foreign policy opposition was regarded as subversive and some anti -interventionists' civil liberties were violated through intensive FBI scrutiny of their political dissention. Moreover, the FBI's surveillance marks the origins of the FBI's role in the later national security state. Among those targets examined in this dissertation include Charles Lindbergh, the America First Committee, notable anti- interventionist senators and congressmen, the anti -interventionist press, and other prominent individuals who advocated American isolation from foreign war.