The United Systems Militias

"'Never underestimate the tenacity of the primitive humans when invading human-held worlds. Nearly every single able-bodied one will take up arms against any invaders, and will fight to the bitter end. No human world can be counted as secured for decades afterwards' Memorandum to Brakian Field Commanders along the Human-Brakian Border"

Overview
For as massive as the United Systems Army is, it cannot be everywhere are once. When worlds come under attack, their first line of defense is often the populace of the planet itself.

These armies are often underequipped, armed with weapons from before and just after the Great Ash Wars, but also outnumber the invaders by several thousand to one, using their sheer numbers to counter their lackluster armaments.

When worlds are "conquered" by the Brakians, and in one case the Zel'gan, the conquers have to contend with crippling guerillas for decades to come, often long enough for the regular United Systems Army to come and liberate the world.