Dropping the atomic bomb on Hiroshima
Here's the chronological order:
Invasion of Poland - September, 1939 (beginning of World War II in Europe)
Battle of Britain - July thru October, 1940
D-Day invasion - June, 1944
Dropping of atomic bombs on Hiroshima and Nagasaki - August, 1945
The use of atomic bombs was the final devastating act of destruction of the Second World War.