Making them fight each other works by making it not fun for them to fight your own country.
Country A has 40 players.
Country B has 40 players.
Country C has 16 players, and 45% of it's own bases left.
Country A and B are attacking Country C, either in a furball, or by going after that one irresistible base that gets them to 35% of Country C. Nothing is happening on the Country A vs Country B front.
Nothing in this universe is going to deter Country A or Country B from what they're doing, except maybe time. We shouldn't delude ourselves into thinking there's a fix for this. Country C's players - if we're going to be real - should just find a spot on the map that they find fun, or simply log off and find something more fun to do. If that player comes back and keeps experiencing that and keeps logging off, then that player might at some point stop coming back, and put his entertainment dollar somewhere else, allowing the market to decide how much this game is really worth.