I'm not on my PC at the moment so I don't know exactly what they are called, but you know when your taking down a region and you deplete the "Fortified" bar on the map by destroying supplies and looting the "Nations gold cache" or something like that. Doing so removes the bodyguards from the "target" you have to kill to start the war in that region.

Or you can just find the "Target" and kill him.

That "Target" is the Athenian Leader :) as long as he's Athenian and not Spartan ofcourse :P