The fact that the initial push is next to impossible without hugely lopsided numbers means that most of the time the map stays as though it just began.
If you win back territory you hit a brick wall as soon as you try to maintain momentum and start hitting native enemy fields.
It's the same situation as the 300 spartans faced when invaded by Persia. A force of 7000 Greeks were able to defeat between 100,000 and 300,000 persians simply by using terrain as a funnel.