The Last Colony

John Scalzi's The Last Colony is the third in The Old Man's War series.
They must save themselves - or die trying.
John Perry was living peacefully on one of humanity's colonies - until he and his wife were offered an opportunity these ex-supersoldiers couldn't resist. To come out of retirement and lead a new frontier world. However, once on the planet, they discover they've been betrayed. For this colony is a pawn in an interstellar game of war and diplomacy. Humanity's Colonial Union has pitched itself against a new, seemingly unstoppable alien alliance, dedicated to ending all human colonization. As this contest rages above, Perry struggles to keep his terrified colonists alive on the surface below - despite dangerous interstellar politics, violence and treachery. And the planet has yet to reveal its own fatal secrets.