Old Man's War universe
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 aliens willing to fight for them are common. The universe, it turns out, is a hostile place.
So: we fight. To defend Earth (a target for our new enemies, should we let them get close enough)