The year is 2020. With relations going strongly between Equestria and Earth, the Princesses allow the United States to set up a military base within Equestria as a gesture of goodwill. The 11th "Blackhorse" Armored Cavalry Regiment, the 3rd Marines, and elements of the 2nd Battalion, 75th Ranger Regiment are sent to act as the first force on the ground. The Forward Operating Base, designated Camp Excelsior, was set up outside Canterlot.
Things are never that simple. Threats surmount Equestria from all sides, including the Changelings, ready once again to conquer, local revolutions from ponies themselves, and of course, inter-service rivalries.
A side story to Boredhooman's A New World, A New ThreatMore Less
Cain’s Years as an independent Commissar were often fraught with not only terror and death, but also of victory and fame. Many of his feats, such as his participation in the Reclaimers’ quest onboard the space hulk Spawn of Damnation and his uncovering of a genestealer cult on the planet of Gravalax that saved the Imperium from the full threat of a Tyranid hive fleet, are well known. However, there are some tales that he dares not tell, either from Inquisitorial order, reluctance to remember such traumatic events, or the belief that spreading the story would discredit him. This is one such case.
-Amberley Vail, Ordo XenosMore Less