World War II was a massive conflict that involved most countries around the world. Hence the name. It’s also very complex, with lots of different elements involved.
However, it is possible to pare World War II down to a couple of basic truths. And the “Li’l Hitler” sketches from Robot Chicken do this superbly.
Adolf Hitler wanted to expand beyond the borders of Germany, and did so by invading countries. The United States wasn’t interested until Japan attacked them, and then they suddenly got involved.
OK, so there’s more to World War II than that, such as The Holocaust which killed six million Jews. But as a starting point this is bang on the money.