Many tales occur in a post-apocalyptic setting, but far fewer in a post-post-apocalyptic setting. This is one of the latter kind of tale. That is, the apocalypse already happened, about 20-30 years ago. It was the big one - armies, nukes, satellite weapons, all of it. On the order of 90% of humanity was wiped out - those who survived were those who had fled to the remote sections of the world that weren't targets during the war, and those who had lived in the relatively safe lunar colonies.
The threat of war was not unexpected, however, and humanity's leaders had prepared contingency plans that would allow them to ride out the war - few, if any, managed to actually use these shelters for themselves, but their discovery by the survivors meant that the rebuilding process had a leg up. Fallout sweepers, devices that absorbed the lingering radiation and made the area safe to inhabit again, soon allowed for the reconstruction of society. The rebuilding process took roughly a decade and a half to return to levels of technology roughly at our own, but it was not a thousand years as many had feared it might be.
And science continued, as the lost technology continued to be rediscovered and reverse-engineered from piles of scrap. Humanity became far more unified than it once had been, with the extreme conflicting ideologies of the previous era wiped out through their own force.
Far more, but not completely, for the world was yet a dangerous place. One faction of humankind survived and rebuilt under the grip of the genius Russian scientist Damon Crow, who isolated his people and bred xenophobia into their culture.
Meanwhile, far across the world, another colony had risen to prominence - Corusa Industries, a corporatocracy formed in the wake of the CEO moving each and every one of his employees and their families to a safe zone prior to the war, and thus forever earning their undying loyalty. Structured in the form of the company itself, with every citizen an employee, they formed something close to a utopia.
It would not be long before this utopia came under attack, by enormous piloted humanoid weapons. After investigation, they were called Xenomorphic Modules - alien life forms encased in armor to control and weaken them. The pilots themselves seemed unaware of this.
The most common variety of XM, the Peska is believed to be the larval form of the species from which the XMs derive. Its armor is light, serving as little more than the restraining device for the alien within. The xenomorph itself is undeveloped, yet still a serious threat in combat - its left arm features a combination plasma saber and mid-performance beam rifle, while its right arm is capable of wielding hand-carried weaponry.
Piloting is done from a cockpit embedded in the back of the alien's torso, just below where the shoulders meet the neck, which is wired into the alien's nervous system, overriding whatever brain it may have.
The Peska's beam saber is a dangerous weapon - its power source is unknown, and is believed to derive from the alien itself. Removal of the armor indicated that this was in fact an organic weapon - whatever species is used to create XMs was designed for war.
Though in its first contact, the Peska was at first thought to be a nearly unstoppable force, Corusa Industries was prepared, as it unveiled its own newest weapon - the Advanced Trooper Kerykeion.