You'd almost have to create a new layer of tile to delimit "controlled warzone" territory from cultural/political territory.
This is because:
How would pillaging work?
Units heal much slower in enemy territory;
In border cities, they could place districts/wonders in these tiles if not treated right;
Similarly, would have to handle what happens to enemy districts/wonders on captured tiles.
For the sake of sanity, I'm assuming these tiles would have to be contiguous- can't just drop in a spec ops unit and steal a hill 10 tiles from the actual front.
I'm not sure if under the hood you can have player owned tiles that don't belong to a city. I could be wrong though.
Conditions wise, you'd also need to think of how this is executed and how to reverse the process. What no one wants is a cavalry unit drawing lines through your country.
Some other thoughts: mechanically one could do this by essentially having a layer that classifies all cultural tiles of a player as one of two types: "frontline" tiles and, for lack of better term, "rear" tiles. The frontline starts at your borders.
Then, whatever the method of actually flipping tiles, it could be such that only player's frontline tiles can actually change hands. If a tile becomes another player's frontline, then his rear tile adjacent to it become his new front line tiles. This way, the front moves organically. Second, one can have different mechanics for gaining or losing frontline tiles. Perhaps we use something like, your army has to exert uncontested zone of control on a tile for a turn to claim it. Using that, it can automaticallyseparate situations like: did your tank zoom through and now it's behind enemy lines, or did a group of your tanks punch through their line to create a bulge?
Then, we could add another mechanic: if your frontline tile is culturally another player's, then it can decay. This way, you have to follow up your blitz with support, lest you end up as a pocket in enemy territory.
Naturally, some kind of healing/movement/"logisitics" mechanic to actually have a reason to control territory vs run around and blast stuff. You could even tweak some promotions or unit attributes so some units (spec ops? light cav?) become behind enemy lines guerrilla warriors, while others (tanks) are specialized in punching a hole through units on a front line (charge) or doing pincer moves when attacking from an enemy's rear tile (rout), while still others (artillery) fight better when shooting from a well supplied "rear" tile.
Just a 5 minute reaction. This idea definitely comes up from time to time and would be a neat mod for a World War scenario. Might have to be careful about making sure land units can only flip land tiles and naval units are need for water tiles ( with some thinking about handling lakes and small bodies of water.) Seems like it might be messy to code in though.