Isolationism, national policy of avoiding entanglement in foreign affairs, a recurrent phenomenon in U.S. history. In 1823 the Monroe Doctrine tried to exclude European powers from the Americas. The United States entered World War I reluctantly, stayed out of the League of Nations it helped create, and entered World War II only when attacked. Thereafter it joined the UN and international defense pacts (NATO, SEATO) and played an active role in international affairs. British policy was essentially isolationist in the period between the world wars.