The United States has been isolationist at many points throughout its history. For example, George Washington advocated a policy of non-intervention in European affairs in his farewell address. At the outbreak of World War I in 1914, Woodrow Wilson, the American president, was decidedly isolationist and hoped to stay out of the war. In fact, he ran on that promise in his 1916 re-election campaign. However, by 1917, the U.S. was involved in World War I.
After World War I, the U.S. again became isolationist. For example, Republican Senator Gerald Nye of North Dakota released a report that stated that World War I had been fought mainly for the benefit of banking interests and arms manufacturers, making Americans skeptical about fighting future wars. The Great Depression of the 1930s kept the U.S. isolationist, as we had our own economic worries to attend to.
It could be argued our isolationist policy was beneficial to us in the years before World War II, as we became involved in the war much later than the European powers and Russia did. We did not begin fighting until December of 1941, while the European conflict had begun in 1939. Europe and Russia had already borne the brunt of Nazi aggression for over two years before American involvement in the war, and the Chinese had been fighting against Japan since the 1930s. While many Americans died fighting World War II, American losses were far fewer than those of the Europeans, Chinese, and Russians, among others. The U.S. emerged from the war as a superpower in part because many European countries such as England and Germany had suffered heavy losses and needed to be rebuilt, and Japan had been defeated in the war and also suffered major losses and damage.
After World War II, the U.S. ended its policy of isolationism because of the Cold War. The U.S. was interested in the policy of containment, or stopping the spread of Communism after the rise of Soviet Russia as a superpower. In the last days of World War II, the Soviets had taken over many Eastern European satellite countries, and the U.S. feared that other countries would fall to communism. This was the rationale for American intervention in wars such as Korea and Vietnam.