That is indeed a big part of the problem. There used to be Marines with a lot of experience in this sort of thing, back in the 20s and 30s of the last century. There's even a nice little book called The Small Wars Manual that was printed at Quantico which discusses the practical problems of conquering and occupying a 3rd world country. The problem these days is that very few Marines have read it, and I doubt anyone in the other services has.