Dog Food.
Pull "Federal" units out--like the Davion Assault and 5th Guards. Pull the Crucis Lancers and Avalon Hussar RCTS out. You are left with a few merc companies, 3 Syrtis Lancer RCTS and the CMM RCTs. You just don't have enough units loyal to such a vast area of space. Even the Outworlds Alliance is going to be eating your worlds, not to mention the Taurians.
Nom Nom Nom.
thus my bit about how they'd have to closely ally with another power. which i'll admit, would be tricky for the Capellan march and the Hasek-Davions.. since the only other nation in the region with enough power to spook the fedsuns away from a military solution would be the Capcon, and a lot of the upper social classes in the region are very anti-capellan..
that said, i suspect that Micheal Hasek-Davion, had he taken the March truely independent as a seperate nation, would have arranged uprisings to boot the davion nobility and the other problematic "carpetbaggers" out.. remember that while the region's nobility and military are davion (mostly from other parts of the fedsuns), the general populace on those worlds are more capellan in general culture, since a lot of those worlds were once the Capcon's.
so after a couple Nights of the Long Knives, his new state would have less qualms about allying with the Capcon. with the Capcon able to then concentrate more of its forces on the other parts of the border, and making motions that they'll invade the Fedsuns if the fedsuns attacks the new state, that would buy Micheal hasek-davion time to draw from militia forces and ideological recruits into new standing army,