I returned after a small absence, and within a very short time what the OP has noted was obvious. This may seem argumentative or maybe too simplified, but "Win the War" is DONE, OVER WITH, and KAPUT.
There is NO mechanism that will prevent free thinking customers from joining a side with advantage.
One possible solution, revamp Melee not to include change in territory ownership. Troops could change a town allegiance but not the airfield / ground vehicle base. Success of routinely timed staged missions, could be used to benefit country status instead of territory. In order to affect the change over of a map, at least 2 cycles of player heavy time zones should be allowed and something quantitative like the # of field strats / buildings damaged / # equipment used / damaged and perks awarded proportionally to all participating chess piece warriors.
Tons of ways to keep the players engaged by tying object / vehicle / plane destruction to rank, hierarchy, equipment or perks, however the mechanism works best.
This plays to the die hard players, as evident in the increase in numbers of lesser advantage sides ramping up to special event attendance,. If players must, then let them go to "capture the sheep".