What Caused World War II to Begin?

One of the most horrific catastrophes in human history was World War II. Existing conflicts around the world became linked, entangling nations in a massive web of violence. It was fought on every inhabited continent by land, sea, and air. Over 55 million people were killed, some as combatants, some as civilians caught up in the conflict, and some as victims of their own governments.
Why did World War II break out?