Lost Fleet：Beyond the Frontier: Steadfast Book 4

Geary and the crew of the Dauntless have managed to safely escort important alien representatives to Earth. But before they can make tracks for home, two of Geary's key lieutenants vanish. The search for his missing men leads Geary on a far-flung chase, ultimately ending at the one spot in space from which all humans have been banned: the moon Europa. Any ship that lands there must stay or be destroyed―leaving Geary to face the most profound moral dilemma of his life.

To make matters worse, strains on the Alliance are growing as the Syndics continue to meddle. Geary is ordered to take a small force to the border of Syndic space. But what he finds there is a danger much greater than anyone expected: a mysterious threat that could finally force the Alliance to its knees.

As Geary spearheads a desperate battle to protect the Alliance against a shrewd and powerful enemy, he's left with just one question: Who are they?