The Capellans are in the position of perpetual underdog in the IS, and for a reason - they're hilariously poor compared to every other Successor State. They're conniving, sneaky bastards.
They're also damned fucking GOOD at sticking around, despite the odds being against them. They suffered through insane and inept leadership, being worked over by having the fucking plot-armored bastards in the FedSuns as neighbors... their literacy rate is bar-none the highest in the entire IS, and while a great deal of their forces are forced to make do with the equivalent of a three-line rifle and a mule cart they tend to be absolutely fucking fanatical (do not fuck with the Death Commandos, just don't, they're completely fucking insane and evil). If the Davions weren't on their border they likely would have already devoured the Free Worlds League, which can't stop hitting itself in the face repeatedly.