They picked a fight with the wrong species.
"A Nation at War": the United Stars of America. Born in the conflagration of unprovoked alien attack, the newest entrant to galactic politics took the few crumbs of hypertech gifted to it and ran with them, soon expanding over dozens of star systems and establishing a wide trade network, protected by its powerful Navy and the dreaded Warp Marines.
"A Fight to the Death": a single Marine platoon, tasked with protecting an embassy on a hostile alien planet. An embassy - and the human enclave around it - that soon finds itself surrounded by armed mobs. Can the Marines and a ragtag band of civilian and Navy personnel survive long enough to be rescued?