How did World War Two start, and what was the cause of it? It happened because five events changed the course of history, and the first one was the attempted assassination of Adolf Hitler. We’ll go into more detail about that later. The USSR got dragged into another war. More official German records on Operation Cobra led to D-Day. Also, Woodrow Wilson's dream of a democratic utopia ended and did not last more than half a century. World War Two started because of one crisis after the next. We’ll then examine the role of Japan's attack on Pearl Harbour.