The American Colonial Guard Corps, or simply 'The Guard', is a branch of the United American Systems military that serves as the first line 'trip wire' force for planetary defense in the frontier and outer colonial holdings. It comprises thousands of common-template planetary defense militias, operating with a semi-independent doctrine with local units under a minimal central umbrella of organization. These forces specialize in localized 'point defense' against planetary invasions via being co-located with their own stores of equipment and being stationed in their own home planetary counties and districts. This allows these units to operate on the proverbial home field and allows them to mobilize in mere hours. That and being engineered for resilient, distributed operations that don't rely on unbroken central command and control.
Manpower draws from a deliberate 'high-low' personnel model
The core mission of the CGC is to serve as a rapid readiness local combat force to either stop or significantly delay a hostile planetary incursion, two methods are employed for this mission. The first being the full activation of a local unit, it's personnel drawing all of their equipment, linking up with their vehicles, and conducting full scale war fighting operations against the invaders. The other option, if orbit is completely compromised and the various means of denying have failed or are delayed a force can opt to operate in a more guerilla fashion. Using the lighter equipment in their pre-located stashes to conduct an insurgency operations. Overall these options are greatly boosted by the unit operating in what is essentially their backyard with a population they are apart of.
Other uses of a CGC unit include augmentation of regular United American Army or United American Marine Corps units for surface operations during widescale planetary warfare, a provisional trained reserve pool for the same said units, and as a civil emergency response force toward conducting security operations.