One of the very best ways to understand World War II is to look at Frank "Capra's Why We Fight," series. Capra, America's finest movie director (It's a Wonderful Life, Meet John Doe, Mr. Smith Goes to Washington, etc.) produced this series for the War Department. It was initially shown to American soldier as part of their training. Later, the films were released to the general republic.
For extra credit, watch any of the episodes linked below. I the comment section below, pick out three or four ideas from the episode that show the strength of the totalitarianism, the initial weakness of the democracies, and/or the way to democracies got their act together. You can also just comment on what you learned from the video you didn't know before.
You may repeat this assignment for extra credit, but, with each video you watch, please submit a separate comment.