Starred Review. Though a lot of SF writers are more or less efficiently continuing the tradition of Robert A. Heinlein, Scalzi's astonishingly proficient first novel reads like an original work by the late grand master. Seventy-five-year-old John Perry joins the Colonial Defense Force because he has nothing to keep him on Earth. Suddenly installed in a better-than-new young body, he begins developing loyalty toward his comrades in arms as they battle aliens for habitable planets in a crowded galaxy. As bloody combat experiences pile up, Perry begins wondering whether the slaughter is justified; in short, is being a warrior really a good thing, let alone being human? The definition of "human" keeps expanding as Perry is pushed through a series of mind-stretching revelations.

John Perry did two things on his seventy-fifth hirthday. First he visited his wife's grave. Then he joined the army. The good news for humanity is that we have finally made it into interstellar space. The bad news is that there are few planets out there fit to live on - and plenty of alien races willing to compete for them. Therefore we must fight: to defend Earth itself, and to stake our claim to planetary living space. Far from Earth this conflict has been going on for decades, brutal, bloody and unyielding.

Earth itself is now a backwater, with the bulk of humanity's resources in the hands of Colonial Defense Forces. But you cannot join the CDF until you reach retirement age because they don't want young people;only those with the knowledge and skills amassed during decades of living. Once you join CDF, you'll be taken off Earth and never allowed to return. First come two years at the front and, if you survive, you'll be given a generous homestead stake of your own on one of those hard-won colony planets.

When John Perry agrees to this deal, he has only the vaguest idea of what to expect. Because, light years from home, the actual fight he faces is far harder than he could imagine.., and what he himself will become is far stranger still.