The actual game mechanics cannot simulate wars or the conquest of a region by another player. Wars are a RP thing, and the key is to making them succeed in RP terms is to have the consent and co-operation of the other players involved. See section 9 of the
Rules of the Game for more information on the rules surrounding military RP.
In practice, war RPs can be challenging to pull off, so it would be a good idea to talk things through thoroughly with the other players first.