The opposition to the United States' entry into the war, Lend Lease, U-boat attacks on Atlantic convoys and American responses, mobilization of America after Pearl Harbor, loss of the Philippines, Doolittle Raid, Midway and Guadalcanal.
Although those in the know figured it was just a matter of time, the United States only reluctantly participated in World War 2. But it was not a foregone conclusion that the United States would side against both Germany and Japan. Japan's attack on Pearl Harbor precipitated a quick decision. But in Germany's case a long series of mutual provocations, some intended, some not, gradually turned the United States against Germany.—David Foss