Best off starting, or I'll lose my momentum.
In the early 2100's, Humanity's single grip on the stars was contested for the first time. 3rd Sector worlds reported large amounts of gamma radiation, but it was put down to solar activity, and no action was taken. 3 months later, urgent distress signals came from all 3rd Sector worlds at the same time, crashing Milset (Military Satelite Network) for hours, thereby disarming the Human Navy. By the time the backups had come online, it was clear that Humanity was by no means alone in the universe. Bitter fighting ensued in the years that followed, and large swathes of the 2nd Sector were destroyed, either to deny Humanity's enemies, christened 'The Entity' by the press, valuable resources, or by the new, star-destroying weapons of this alien race.
The fighting bludgeoned its way to a brutal stalemate after 50 or so years of non-stop fighting. It was only held this way due to constant sacrifice on the part of Humanity, who continually route billions of unprepared soldiers to the frontier, to drown the fighting in tides of blood and death.
Humanity's only hope lies in the newly created Shadowheart Program, a research project which uses salvaged Entity technology to improve upon current military weapons, training and methods. The main focus of the Shadowheart Program is hit and run, deep-cover intel gathering and high-profile assassination, all without detection.
The subjects of the Shadowheart Program, appropriately dubbed 'Shadows' are formed into groups of 4 and sent out on Classified Black Operations, nicknamed 'Heartbreakers' due to either the tragic loss to the enemy, or the death of all shadow operatives.
The Shadowheart Program is the only thing keeping the balance in this one-sided war of worlds, ideals and races.