World War II started because there were some unsolved problems from World War I. World War I left many nations crippled, low on supplies, and in debt. The United States isolated itself and did not become involved in European affairs following World War I. Germany was close to economic collapse and had war debt it could not pay. Germany turned to Adolf Hitter to save them and avenge Germanys defeat. Soon a more terrible world war would darken Europe.
On September 1, 1939 Germany invaded Poland and this started World War II.