How did WWI contribute to the beginning of WWII?

The Treaty of Versailles placed blame for the war on Germany and required that they pay reparations...(pay for the mess they made). This devastated the German economy, not to mention the moral of the people. As a result, Hitler came to power by promising to restore Germany to its former strength and status...( I over simplified, but that is the basic idea). The key lesson or thing to wrap your head around is the danger of demoralizing / handicapping an adversary to such a degree, that they are willing to follow anyone who promises to make things better.