timmmay:

 

Forever War Series, Joe Halderman. Anything he wrote is generally good.

 

 

 

John Scalzi has a series that explores similar themes. Old people on Earth can choose to sign up to the military. They are downloaded into a new, upgraded soldier body based on their own DNA, serve their time, and if they survive it, they get downloaded again into a non-militarised version of the body to effectively get a second life. But they can never go back to Earth. In this way, the alliance that controls access to space gets a steady stream of volunteers who are not only willing and eager (because they're elderly and will die soon anyway) but experienced with a healthy appreciation of their own mortality (unlike an 18 year old kid) and by not letting them return to Earth afterwards those people with experienced minds in young bodies are perfect candidates for building new planetary colonies.

The books follow one particular soldier who keeps getting sucked back into the conflicts because the powers keep manipulating circumstances so that it is always the right thing for him to do. He knows he's being manipulated, but has to go along anyway.