The events in Europe that lead to the start of World War II can be broken into three major categories; The German economy, the rise of Benito Mussolini in Italy, and the civil war in Spain.
After World War I, the German economy was crushed and the Germans were still paying the reparations to both Britain and France. Adolf Hitler takes advantage of this situation and of the German people. He plays on their emotions and becomes the Chancellor of Germany. The first action he takes is to stop reparations to Britain and France. Then he starts to remilitarize Germany which boosts the German economy ten fold. The German people love and support Hitler. In 1935, Hitler sends troops into the Rhineland. Hitler's invasion were bluffs that worked because both Britain and France had a policy of appeasement. As Hitler's army grew the more land that he wanted to repatriate because Hitler believe that the people that live in the invaded countries were of German descent. By Hitler using this method, he acquires the Sudentenland of Czechoslovakia with approval from the Munich conference. Six months after the Munich conference, Hitler's army invades the rest of Czechoslovakia. In August 2,1939, Hitler and Stalin sign a non-aggression pact called the Nazi-Soviet pact. Then he demands the Danzig area of Poland. Britain and France protest and promise to protect Poland. On September 1, 1939, Hitler invades Poland the starts World War II.
Meanwhile, Benito Mussolini has been elected in as the leader of Italy. Mussolini then invades Ethiopia in May of 1935. There was a small Ethiopian army resistance which the Italian army with its machine guns and bombers crushed easily. The League of Nations condemned Italy's action which was basically a slap on the wrist. In 1936, Mussolini joined with Hitler and formed the Rome-Berlin Axis. .
Also in 1936, Civil war brakes out in Spain. General Francisco Franco revolts against the established government of Spain.