The harsh terms in the Treaty of Versailles (which ended World War I) help to explain why Hitler was able to rise in power and the start of World War II.
The Germans were forced to pay billions of dollars in reparations to countries like France after World War I and they were also forced to take full responsibility for starting the war. This caused a huge economic depression in Germany during the era known as the Wiemar Republic. This ruined economy allowed for the rise of Hitler, as he promised to restore Germany to their former greatness.
Hitler's rise to power lead to his invasion of several different European countries. These acts of aggression lead to the beginning of World War II.