How did WWI contribute to the beginning of WWII?
The Treaty of Paris, which ended World War 1, actually played a large part in setting the stage for World War 2.
Humiliated in defeat, crushed by debilitating war reparation payments, and angered by the war guilt clause, Germany came out of the war a truly weak nation. Not only did they suffer the most dead during the war, but now they had to deal with all the pain of the peace treaty. The economy collapsed, caused in huge part to the payments Germany had to make to the Allies as a sort of compensation. The treaty forced Germany to admit to causing the war, even though there were plenty of other people involved - further angering the German people.
The worldwide Great Depression of the 1930s essentially nailed the coffin for postwar Germany. Their already brutal economic situation, caused by perhaps the worst inflation in history, became worse. Unemployment skyrocketed. The German people needed a leader to bring them out of the hole, who they found in Adolf Hitler. He stoked the people's fire, appealing to their anger from defeat in World War 1 and their hopelessness from their financial crisis. He eventually rallied enough support to take over the government.
Hitler rebuilt the military and abused his power, both at home and abroad. He used the threat of another conflict to extract fear and compliance from nations like England, whose prime minister signed the Munich Agreement giving Hitler more territory. Continuing with his promises and highly successful propaganda campaign, Hitler's support ballooned, allowing him to evoke nationalistic and warlike sentiment in the German population.
He stirred the German people by telling them their loss in WW1 was caused by backstabbers at home like the Jews, and the terms of the treaty were unfair. The Germans deserved better, he claimed, and he could give it to them. What he was really doing was using Germany's loss and subsequent hardships to get the German populace to support his aggressive foreign policy, and eventually, World War 2. It was not entirely unfounded, however, because Germany did suffer greatly because of the treaty, and it was this suffering that made them hunger for World War 2.