This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of American History. Please check back later for the full article.
For the United States, isolationism is best defined as avoidance of wars outside the Western Hemisphere, particularly in Europe; opposition to binding military alliances; and the preservation of national autonomy. Until the controversy over American entry into the League of Nations, isolationism was never subject to debate. The United States could expand its territory, protect its commerce, and even fight foreign powers without violating its traditional tenets. Once President Woodrow Wilson sought membership in the League, however, Americans saw isolationism as a foreign policy option, not simply something taken for granted. Its high point came between 1934 and 1937, when Congress, noting the challenge totalitarian nations posed to the international status quo, passed the Neutrality Acts to insulate the country from global entanglements.
Once World War II broke out in Europe, President Franklin D. Roosevelt increasingly sought American participation on the side of the Allies. Isolationists unsuccessfully fought FDR’s legislative proposals, beginning with the repeal of the arms embargo and ending with the convoying of supplies to Britain. The America First Committee (1940–1941), however, so mobilized anti-interventionist opinion as to make the president more cautious in his diplomacy.
If the Japanese attack on Pearl Harbor permanently ended classic isolationism, by 1945 a “new isolationism” voiced suspicion of the United Nations, the Truman Doctrine, aid to Greece and Turkey, the Marshall Plan, the North Atlantic Treaty Organization, and U.S. participation in the Korean War. Yet, because the “new isolationists” increasingly advocated militant unilateral measures to confront Communist Russia and China, often doing so to advance the fortunes of the Republican Party, they exposed themselves to charges of inconsistency. Since the 1950s, many Americans have opposed various military involvements—including Vietnam, Iraq, and Afghanistan—but few envision a return to an era in which the United States avoids all commitments.