The UKUS Marine Corp
The UKUS Marines are a pre-Fall military organisation, associated with something called Project Nightfall which appears to have been a plan to place soldiers in cyrogenic storage around the time of the Fall in case they were needed later. The long term goals of Project Nightfall are unclear.
The FEF's first encounter with a UKUS Marine was Captain-Paladin Citadel. He had been sealed in some kind of cryogenic pod and buried. The FEF picked up a distress signal when the cryo pod suffered a catastrophic failure. Those present freed the Captain-Paladin from the pod and revived him with nanite patches. Not much detail was forthcoming but it seems the Captain-Paladin had been buried alongside several other of his corps with the goal of being revived when needed and tasked with hooking up with the expected FEF forces of whatever bunker they were near. He refused to elaborate much further, insisting he would only speak to someone with appropriate security clearances which no-one present had. Captain Citadel was integrated into the FEF but on the understanding that he would leave when he received orders from the correct chain of commands.
During a surface mission to eliminate a powerful tribal leader called Slasher, a PADD was recovered containing the following memorandum about UKUS Marine activity, including the hypnotic manipulation of Warlord Raze for their own ends. Despite the initial assurances from Captain Citadel it would appear the UKUS Marines are not the upfront force for good we were told.
It is strongly suspected that Project Nightfall have a base of operations somewhere in the Pennines with a large number of other UKUS Marines, possibly also in cryogenic storage.