From 1942 to 1945, however, America found itself at war both in Europe and throughout the Pacific. Quickly, isolationism seemed to be a distant memory. In the Pacific theater, the United States declared war on the Empire of Japan following the attack on Pearl Harbor. The United States later demanded an unconditional surrender from Japan, a decision which some argued would only prolong the war. Those who held this position felt that Japan would fight far longer and fiercer if there were no possibility of compromise. Unconditional surrender, however, was adopted as a policy.
This surrender was brought about by the atomic bombings of Hiroshima and Nagasaki, which followed the obliteration of Japans largest cities by conventional bombs. Many argue that the real intention in Americas nuclear bombing of Japan was to serve as a warning to the Soviet Union, Americas wartime ally but soon-to-be adversary. America also wanted to force Japans surrender quickly so that the Soviet Union would not invade and occupy northern Japan. If that had happened, the occupation would have been divided between the Soviets and the Americans, much like what happened in Germany.
By forcing Japans surrender before the Soviet Union could commit large number of forces there, America ensured that Japan would be firmly in the American post-war sphere of influence. In Europe, America adopted an identical policy as it had in the Pacific; the war would not end until Nazi Germany surrendered unconditionally. In the European theater, America fought with allies far more than in the Pacific. As American and British forces closed in on Germany from the west, the Soviet Union advanced from Germanys east. Eventually the Americans and Soviets would meet in Berlin, which had been captured by the Soviets.
Much as in Japan, America had worn down Germany with carpet bombing of many of Germanys largest cities. After Germanys surrender in the spring of 1945, American set about planning its postwar occupation. The primary concern for the Americans was to ensure that the Soviets did not advance any further west than Berlin, as the Americans feared that the Soviet Union was an inherently aggressive nation that would roll over western Europe, just as Hitler had done, if the United States did not deter them with military forces and political support for its European allies.
Almost immediately after World War II, the Cold War began as the Americans and Soviets consolidated their influence over areas of Europe that they had liberated from the Nazis. The United States sought to bolster its European allies by military alliance in the form of NATO and by economic aid in the form of the Marshall Plan. The years 1941 through 1946 represented the largest shift in American foreign policy in the history of the country.
America went from a relatively isolationist country to one which produced half of the worlds wealth, had military personnel stationed all across the globe, and entered into official foreign alliances for the first time in its history. After the war, there was no return to isolationism, which is evidenced by the fact that American soldiers are stationed in Japan and Germany to this day.