Why did World War II begin?

World War II began on September 1, 1939, when Germany invaded Poland. Great Britain and France responded by declaring war on Germany, beginning a global conflict that would last until 1945. Over the next year, Nazi Germany would conquer almost every nation in Europe, and, along with Italy, drive through North Africa.  Hitler also invaded the Soviet Union, with whom he had signed a non-aggression pact, in June of 1941. The war became truly global in December of 1941 when Germany's ally Japan, already having invaded China years earlier, attacked the United States naval base at Pearl Harbor, Hawaii. As the United States declared war on Japan, Hitler responded by declaring war on the United States. Thus all of the world's largest nations were involved in the most destructive war in human history, one which would only end with the utter destruction of Germany, Japan, and many other nations.