With the outbreak of war in Europe, a growing fear of and ultimately a concerted effort to defeat Adolf Hitler and Nazi Germany defined American involvement. Competing Allied national and strategic interests resulted in serious debates, but the common desire to defeat the enemy proved stronger than any disagreements. President Franklin Roosevelt, understanding the isolationist sentiments of the American public and the dangers of Nazism and Imperial Japan perhaps better than most, carefully led the nation through the difficult period of 1939–1941, overseeing a gradual increase in American military preparedness and support for those standing up to Nazi Germany, as the German military forces achieved victory after victory.
Following American entry into the war, strategic discussions in 1942–1943 often involved ambitious American military plans countered by British voices of moderation. The forces and supplies available made a direct invasion of northern France unfeasible. The American desire to launch an immediate invasion across the English Channel gave way to the Allied invasion of North Africa and subsequent assault on Sicily and the Italian peninsula. The Tehran Conference in November 1943 marked a transition, as the buildup of American forces in Europe and the overwhelming contribution of war materials enabled the United States to determine American-British strategy from late 1943 to the end of the war. The final year and a half of the war in Europe saw a major shift in strategic leadership, as the United States along with the Soviet Union assumed greater control over the final steps toward victory over Nazi Germany.
By the end of World War II (May 1945 in Europe and September 1945 in Asia), the United States had not only assumed the leadership of the Western Allies, it had achieved superpower status with the greatest air force and navy in the world. It was also the sole possessor of the atomic bomb. Even with the tensions with the Soviet Union and beginnings of a Cold War, most Americans felt the United States was the leader as the world entered the post-war era.