Oblivion

From the introduction:

Welcome to 2057. Fifty years ago, a race of aliens known to us as the Scavs destroyed the moon. This was their opening move, and set of a chain reaction of gravitational forces that caused earthquakes the world round. Humanities counter attack was nuclear, and ended up destroying what was left of the world. Here and now, humanity has migrated to Titan, one of Saturn’s moons. Only a few thousand people are left on the planet, the mop up crew, guarding the fusion reactors that power the Tet, and by extension, Titan, from the last of the Scavs.