Invincible by Jack Campbell

Admiral John "Black Jack" Geary was revived from cryogenic sleep to lead the Alliance against the Syndicate Worlds. But his superiors question his loyalty to the regime. Now in command of the First Fleet, he is tasked with exploring the frontier beyond Syndic space, a mission he fears deliberately puts the fleet-and himself-in harm's way... An encounter with the alien enigmas confirms Geary's fears. Attacked without warning, he orders the fleet to jump star systems-only to enter the crosshairs of another hostile alien armada. Ignoring the First Fleet's peaceful communications, this species sends its ships into battle while it guards the exiting jump point with a fortress of incalculable power. Now Geary must find a way to reach the jump point without massive casualties-even though the enigmas could be waiting on the other side.