i would recommend the reputation system from jumpgate: classic...
best solution i ever saw for these kind of problems...
if u want to check it out, play it for free @: http://jumpgate-tri.org/
every player has reputation to every nation in the game and by attacking players/npc of another nation u got minus rep and there nation turned hostile to u... if a lot of player did this, your nations turned hostile to each other... and of course u also got a small bounty on your head by attack a non hostile nation... if they were in war, there was no bounty
if u wanted to end your own hostility or the war, u were able to perform missions for the hostile nation (and of course every other in the game) to gain positiv reputation towards them...
so if a lot of player decide to do special missions for another nation, u were able to get an alliance with this nation...
there were no diplomatic system... the game reacts on how the players play the game... for me by far the best system i ever saw in over 17 years of mmo's
cheers, abuu