What made WWII such a deadly war?

It began when Nazi Germany unleashed ferocious attacks across Europe - but it spread to the Soviet Union, China, Japan and the United States. Cities were destroyed by air raids, the atom bomb was dropped on Japan and six million Jews were killed in the Holocaust.

Explanation:

Hitler had long planned an invasion of Poland, a nation to which Great Britain and France had guaranteed military support if it were attacked by Germany.On September 1, 1939, Hitler invaded Poland from the west; two days later, France and Britain declared war on Germany, beginning World War II.