Exultant

For more than twenty thousand years, humans have been at war with the alien race of Xeelee. Faced with certain death, a young pilot, Pirius, disobeys orders and travels into the future. Upon his return, Pirius is court-martialed and sentenced to penal servitude. But it is not only Pirius who pays the price. In flying into the future and back again, Pirius returned to a time before he'd left, a time inhabited by his younger self, who also receives punishment.

Commissary Nilis believes that the elder Pirius, whom he dubs Pirius Blue, may know how to defeat the Xeelee. But Nilis can do nothing for Pirius Blue. Instead, he takes the younger Pirius--Pirius Red--back to Earth. There Pirius Red will discover truths that shatter his preconceived notions of all that he is fighting for, while Pirius Blue will learn even harsher truths. But the most shocking revelation of all is still to come.

This is #2 in Destiny's Children Series and #7 in Xeelee Sequence Series.