Why was world war 2 important?

War officially began on September 1, 1939, when Germany attacked Poland ... Japan's plans for expansion in the Far East led it to attack Pearl Harbor in December 1941, bringing the United States into the war. By early 1942, all major countries of the world were involved in the most destructive war in history.

Answer:The pact with Stalin meant that Hitler would not face a war on two fronts once he invaded Poland, and would have Soviet assistance in conquering and dividing the nation itself. On September 1, 1939, Hitler invaded Poland from the west; two days later, France and Britain declared war on Germany, beginning World War II.