Answer: Germany turned to fascist leaders who rallied the country around nationalism. This, in turn, led to World War II.
Details:
The harsh terms of the Treaty of Versailles, coupled with the effects of the Depression, put Germany in a situation that allowed the rise of Hitler and the Nazis, leading to World War II.
The Treaty of Versailles, which came out of the Paris Peace Conference in 1919, was very punitive towards Germany. Germany was forced to admit responsibility for causing the Great War (World War I). We now call that "the war guilt" clause of the treaty. Germany also was forced to pay large reparation payments to the llies (who opposed Germany in the war). Germany also had to give up large portions of its territory and lost colonial territories. And the German military had major restrictions imposed on it -- it had to be a volunteer military only, of no more than 100,000 men, and they could not have an air force.
The German economy and national pride were deeply wounded by the terms imposed by the Treaty of Versailles. Coupled with that, the Great Depression (which had worldwide impact) made the economic situation even worse. The bad situation in Germany made it possible for a radical leader like Hitler, making all sorts of bold promises, to win over enough people to rise to power. Hitler promised a return to national greatness and fiercely rebuilt Germany as a military machine. The rise of Hitler and the Nazis brought about World War II in Europe.
