Lost Fleet - Beyond the Frontier - Steadfast Book 4

Strains on the Alliance grow as the Syndics continue to meddle, and Black Jack Geary is ordered to take a small force to the border of Syndic space. There he finds a mysterious threat that could finally force the Alliance to its knees. As he mounts a desperate battle to protect the Alliance against a shrewd and powerful enemy, he's left with just one question: Who are they?