Oh I got an idea..
What killed off most of the big invasions in history? Long supply lines..
What I mean is that a faction sending out an army.. well it may starve or start to "loose equipment" or such, if it is hanging out a certain amount of hexes from a friendly/allied town. Let's say an army is 5 hexes away from nearest town.. penalties start to apply to it.
Now.. if an enemy were to cut off the shortest way of supply by occupying hexes behind it. Either it gets cut off completely, and begins to really starve fast, or the shortest route to a friendly/allied town is now 10 hexes, and a larger penalty applies!
This is only realistic, and must be implemented!
(And it makes sense, and it would create more strategic depth, and armies would have an incentive to attack their neighbors before that small vulnerable faction on the other side of the map.. Such a venture is of course possible with allies though.)