John Perry did two things on his 75th birthday. First he visited his wife’s grave. Then he joined the army.
The good news is that humanity finally made it into interstellar space. The bad news is that planets fit to live on are scarce—and alien races willing to fight us for them are common. So: we fight. To defend Earth, and to stake our own claim to planetary real estate. Far from Earth, the war has been going on for decades: brutal, bloody, unyielding.

Scalzi has imagined an interesting universe and there are a lot of intense action sequences balanced by thoughful, nuanced passages. The book also has a lot of clever, salty dialogue between the soldiers that is often quite funny.