How did World War 2 start?

There were many events leading up to WW2. These include the Spanish civil war, the Japanese invasion of eastern China, the Soviet-German (Molotov-Ribbentrop) treaty, the Nazis' rise to power, the German annexation of the Sudetenland, and more.

Then, on 1 September 1939, Germany invaded Poland. In response to that invasion, Britain and France (who had a treaty with Poland) declared war on Germany on 3 September 1939.

After the conquest of Poland, Germany continued with blitzkrieg (swift) conquests of France, the western USSR, and the rest of Europe.