What caused World War II ?

Most historians believe that the causes of World War II can be traced to World War I (1914-1918). Americans had fought in that earlier war to "make the world safe for democracy." Those were the words and goals of U.S. President Woodrow Wilson. But the peace treaties that ended World War I did not make the world safe for democracy. Instead, they caused bitterness and anger that led to World War II.
Germany and its allies had been the losers in World War I. Germany was stripped of one sixth of its territory and forced to pay huge reparations (payments by a defeated country for the destruction it caused in a war). After World War I, Germany suffered from high unemployment and runaway inflation. German money became almost worthless. Many Germans seethed in anger at the peace treaty.
A League of Nations was set up after World War I to keep the peace. But the U.S. did not join, and other countries were too busy with their own problems to worry about Germany and other trouble spots.
Then, in the early 1930s, the world was hit by an economic depression. Workers lost their jobs, trade fell off, and times were hard. People looked for leaders who could bring about change.