[Hugh Manatee]

Or they could have a reputation system, like in guild wars. When you complete a mission or fight for either side you get "faction" for that side, the more you have for that side the more they like you, and if you have more faction for 1 side then the other, the other side will hate you, but at any time you can do quests missions or battles for either side to build you reputation. The catch is where the rewards and consequences of fighting for any given side, including access to special armor, merchants, missions, and NPC's that will offer you services in explorable areas. If you are in hostile territory, you won't be able to buy the armor, use the merchants, and you have to fight or bribe the NPC's that would have helped you, and as said the border changes based on who's winning the constant battles that are taking place.