World War II was fought, in some way, on every continent on the planet except Antarctica.  The total number of human lives lost in the War will never be know for sure but historians believe the number to be between 75 - 80 million (US Census World Population, 2016) which is about 3% of the world population.  Of those lost, the overwhelming majority were civilians or non-military personnel.  It is by far, the largest human conflict in the history of man.  

The United States was somewhat lucky for no battles were fought on US soil.  The United States will enter the War as a somewhat isolated country and will emerge four years later as the strongest country on earth.  World War II had to be fought, but it's impact on world events will forever alter the progression of man.