How did World War 2 happened?

A complete answer would fill several volumes. The short answer is that Hitler had been successful in bluffing the Western Democracies into making concessions to his expansionist policies and did not think that they would declare war if he moved against Poland, especially if the USSR attacked from the opposite direction as part of their Non-Aggression Pact. Yet, that is what happened. When he invaded Poland, Britain and France declared war on Germany. On the other side of the globe, Japan who was already involved in a war with China, came under threat of embargo by Holland, England, and the United States when Japan decided make surprise attacks against them and knock them out in the first days of a wider war. Japan struck Pearl Harbor, the Philippines, Hong Kong, Singapore, and the Dutch East Indies in December of 1941, opening the Pacific Theater of WWII.