The Front-Line Team: Book 3D

Ronan chooses to go to space with the Front-Line Team, whose job is to try to stop the vephronian attack and keep the peace. When he leaves, he finds that the universe is not as he thought. There are many conflicts and universal war could break out if he can't help prevent it. Humans are not as strong as many of the races he learns of, so if he can't protect them, they may fall.

As all books in this series, it's a Pick Your Path book, and you get to decide where the story goes at the end of the book.