Basically, it's because YOU are the big, bad, 'evil empire' and absolutely must be stopped. Makes perfect sense.

I'm in two minds though. Another type of logic suggests that more alliances with you should be sought, in that they seek alliance as their only hope of protection from the big power. Sucking-up to the big bully, to put it bluntly. However, this is putting great store in diplomacy, matters of honour and the fickle whims of a single individual - the great dictator.

There ought to be some heavy penalty on influence rating for despicable acts like attacking an ally, particularly if it was one of long standing. Knock-on effects on loyalty at home too, obviously. Although the game manual hints at 'loss of honour' for doing such things, I can't say I've seen any detrimental impact, either for my faction or for AI factions doing this sort of thing.

Then again, if you're playing the Conquest game, surely you want this to happen anyway? Once you have no allies, you are free to attack in every direction, without any loss of honour. (Just don't expect to secure any royal marriages ever again....)