The world has narrowly averted ecological disaster.
Aliens arrive, heralded by a massive electromagnetic pulse - whether this a side effect of their Faster than Light drives or a deliberate attempt to blind the military forces of Earth is lost to history. But a counter-attack is launched, and in the short war that follows, nations are broken apart a bio-weapon was unleashed leading to the rise of "Reanaimates" clinically dead yet still moving corpses with a desire to consume the living and often distressing mutations.
Half a decade after the event, humanity has passed its initially struggle to survive, and is now in the era of rebuilding. Much of the landscape is now dominated by either city-states, or is the unclaimed "Anarchy Zones". Every leader is sending out scouts to find supplies in now abandoned and zombie over-run cities seeking seeking supplies and technology to cement themselves as new nations in the changed world.