Guardian (Lost Fleet: Beyond the Frontier Series #3) by Jack Campbell

Admiral Geary’s First Fleet of the Alliance has survived the journey deep into unexplored interstellar space, a voyage that led to the discovery of new alien species, including a new enemy and a possible ally. Now Geary’s mission is to ensure the safety of the Midway Star System, which has revolted against the Syndicate Worlds empire—an empire that is on the brink of collapse.

To complicate matters further, Geary also needs to return safely to Alliance space not only with representatives of the Dancers, an alien species, but also with Invincible, a captured warship that could possibly be the most valuable object in human history. Despite the peace treaty that Geary must adhere to at all costs, the Syndicate Worlds regime threatens to make the fleet’s journey back grueling and perilous.

And even if Geary escorts Invincible and the Dancers’ representatives home unharmed, the Syndics’ attempts to spread dissent and political unrest may have already sown the seeds of the Alliance’s destruction...