WWII would prove to be the biggest, by far, of any war inthe history of the world. The dangers and difficulties were so great that, going in, no one could say how long it would take and how many lives it would cost. So,the government got a line on all the potential manpower by having even middle-aged men register. There was also our experience in WWI. US forces lost about 115,000 men total. Most of these men died between May and November 1918.