Joe Haldeman 
Forever War #6 [PDF ebook] 

Alone in the universe, and still a soldier, Mandella awaits his next interaction with the Taurans, whiling away the days organizing his troops. In his commanding role as Major, one he feels supremely unqualified for, the decisions he makes will result in death and destruction. But with no other place for him in the universe, he has no choice but to perpetuate the violence that has stolen his future, his love, and his life from him, in the vague hope that this Forever War will one day come to an end…