Also, landings will tend to be funnelled to locations where you can quickly get supplied (Ports) and have the opportunity for expansion of the landing area (Ports connected by roads or rail to inland cities). So while technically, you can land anywhere, it will probably come down to certain areas that will see the most invasions.
If you land and take a location in the middle of a swamp with no roads, it could take you over a month just to get your troops out of the area, and all the while they're suffering from attrition, so when you do finally get out, the enemy has had time to reinforce and his troops will be much fresher. Chances are, you're not getting out of that swamp without help from somewhere.
