As I recall, the earliest political murder in Rome occurred because of a form of political factionalism when conservative elites (dominant landowning Senators) were pressured by political challengers to provide land to returning soldiers. This problem eventually contributed to the fall of the Republic-
http://www.unrv.com/empire/gracchi-brothers.php
If you have a highly factionalized army and competing elites, that might put a damper on an expansive KFS, especially if the problem is more about elite rivalry, with powerful political patrons supporting military clients, that trying to keep your military compliant. One might add the danger of a slave revolt and the temptation is to keep the army close (especially if it is small) and protective of the patron. If the army does become expansive- as elites expand their power base- than they need to seize land or people quickly so that they can move back home should the patron be threatened.
The puzzle is why stay together and not just break apart into rival states. An oligopoly of paranoid elites would require an incentive to remain cooperative and willing to accept the risk of coup or assassination. If everyone is in the capital, than they are constantly at risk of assassination or a coup that wipes out the potential rivals and places one as leader. it might be easier just to carve out a chunk of the state, declare independence and defend your chunk of land against adversaries or launch a civil war from outside the capital and try to defeat your enemies.
This is the problem of coups in Africa- you can't break away because the state is either too small, the state as administration is too powerful to abandon. Add to that is the problem that the international community frowns on secessionist movements. The Glorious Revolution replaces the king with parliamentary oligopoly but they had to worry about foreign threats.
SO what keeps the KFS together?