I've been learning a lot about WWI recently, and on of the things that was really interesting to me is how constrained the armies seemed to be by the fact that they had to walk everywhere they wanted to go. Toward the end of the war, it seems like troops were being transported by vehicle in some circumstances. I'm curious when we reached the point where movement was all done by vehicle.
Behind the lines, movement would mostly have been by rail even in the 19th Century.
Regarding tactical mobility, only a minority of troops were motorized even in WWII. The US and Britain went furthest in terms of motorization, but even they could only motorize around 1/3rd of a typical infantry division at once. Other countries' troops had to walk apart from a handful of fully motorized units.
The constraint was simple: money.