"When you think of all the logistics behind an army, it's mind-boggling."
It sure is, just think of the planning involved in any of the amphibious assault landings in the Med & Europe. What troops to load on on which transport? Which vehicles go on each LST & how to "Combat Load" it so the equipment & supplies needed first in the assault come off the LST first? How did they get the troops & vehicles from their training or bivouac areas, sometimes hundred of miles away, to the correct ship at the right time? and to insure the smooth flow of followup troops & supplies to the front lines? It was definitely an awesome task. It took a lot of cooperation between the Navy, Army Transportation Corp & Traffic control HQ and carried out by the Military Police.
Here is a video of how they did it during the Normandy invasion: