D-Day By the Numbers, By the Men

D-Day by N/A is licensed under Good Free Photos Public Domain
Now imagine you have to move all those people and all that stuff partly by air, but mostly across heavy seas in foul weather.

Under enemy fire.

I should also mention that if you messed up any of the big details, a lot of your people are going to die, and then you're going to have to figure out how to move them all back without getting too many more of them killed.

And all that is just the beginning. Because once you've done all that, those men on that "hunting trip" are going to have to take and widen a beachhead big enough and secure enough that you can rebuild (or build from scratch!) the ports and roads necessary to bring another million men over... plus all the additional stuff all those additional men will need.