This is an easy problem to solve and has been done in many community based mmo's, dont make a limit on who an alliance can declare war one, take it to the individual player, so that you can only attack/ be attacked by sum1 25-40% less than you in prestige or you can base it on military power and for those who like a challenge 50-100% prestige above you, that way, the system put in place should allow retaliation by players hit by those who attack them ofc.