Most tbs games have very odd ways of controlling borders, and this is a major flaw in many tbs games imo. Yet I don't know of any undeniably better way of doing it.
No border. Many games, very realistic but unimmersive and boring.
Culture/Influence spreads in circles from cities. Civ 1-4, GalCiv. Unrealistic, but easy to steer.
Regions. TW, EuUniversalis(only played the first though), HoI, and a bunch of others. Realistic, yet unchanging and limits the timeframe. Suddenly capturing 200sqkm of land is also iffy.
Culture/Influence spreads gradually to the land that is the easiest to capture. Civ 5, from what I have seen of it. A bit realistic, yet why does it depend on culture? Can't you take land? And can you with diplomacy trade land?
If anyone can think of others, please do add them.
How it works in real life: (From what I have grasped fro history.)
Zones of control are divided into several things:
Military control: Ie whose army has the most bang, this is something every leader considers yet they are free to act.
Domestic control: The land in which you are the financial leader, everyone buys your goods, reads your books, yet this doesn't directly change politics.
Political control: National borders. This is my land, this is my rock, that will be your rock. Etc. Derives from what you army can protect and how well communications work. This has nothing to do with who has the best artists or the best polished cities.
Cultural control: Who dictates the general thought, how you should live, how you should eat, and this only affect where everyday communication can reach.
Games tend to go for option nr 4 while 1,2, and 3 are even more important in reality. Military might, commerce, and politics should say more in how borders change than point 4.
About the problems with putting it into a game:
Regional control is fine to a point, and reflects reality much better, yet it limits the timeframe heavily, since region games don't want to change or add new regions. In a way all gridbased games are regions, but it always swings around the cities. Only 1/10 (give or take a lot since I pulled this out of my elbow) of the people lived in cities at the start of the 19th century. And while those do affect more than any one tenth of the country, they do definitely not solely decide a nation. You have far from lost if you lose a city in real life. You have lost if you lose your army.
I did a sketch on a way to make tbs games more realistic without going HoI, yet I can't find it now. It would still be boring in many aspects. For one, you wouldn't chose what to research until mid game, everything would be breakthroughs and technology diffusion from distant lands. Yet on many aspects it would still be a game, a prefectly playable game that would have immersive qualities as the world would grow by itself, you would be tending a nation, isntead of a bunch of cities.