How did the events ending World War I bring about World War II decades later?

Two events in particular at the end of World War I helped bring about World War II two decades later. Far and away the most important was the Versailles Treaty. The treaty forced the Germans to assume guilt for World War I. This humiliated and angered the German people, leading to the kind of discontent that allowed a demagogue like Adolph Hitler to rise to power. Second, the Germans were angered at having to pay heavy reparations for the war, mounting to more than two trillion dollars in today's money. Further, the treaty restricted the Germans to one hundred thousand army troops and forbade the Germans an air force. The armed services had been a chief way young men from humble roots could get ahead in Germany. An army career was respected and led to decent pay, so a man could marry, buy a home, and support a family if he chose that path. When this career option was cut off, many unemployed, alienated young men joined the right-wing freikorps that morphed into Hitler's brown shirts.
The formation of the Weimar Republic at the end of World War I also helped lead to the Second World War. The republic was devised hastily as a way to fend off a threatened communist takeover similar to the one that had recently occurred in Russia. However, large segments of the population did not support the idea of a republic, preferring a monarchy. The Republic also lost support as it was perceived as both decadent and weak. Many people wanted a strong absolute ruler and support for traditional values.
Both the resentment wrought by the Versailles Treaty and the weakness of the Weimar Republic gave Hitler his opening to seize power. Hitler's ascent to absolute power is key to the start of World War II: even in Germany, most people wanted to avoid another major war. Hitler, however, was determined toward conflict and conquest.