Lessons of War. Si Vis Astra, #1

An untried captain. A battle-hardened marine. A hidden enemy. As a boy, Ian Hadrian used to look up at the night sky and dream of commanding a starship. As a man, he's one of the Interstellar Compact Navy's youngest battlecruiser captains-but dreams rarely match reality. His command, the Belisarius, is a star-leaping stallion leashed to a solitary and remote sun, watching over a long-suffering world the enigmatic Ciphers abandoned after the Great War.
Far from straining at his leash, Hadrian has come to accept his posting, even at times to enjoy it-perhaps because the heroic official record behind his rapid promotion to captain hides a much darker truth. Whatever Sorana Mirra's childhood dreams were, she's long since forgotten them. Twenty-five years as a marine will do that. Mirra commands a team of marine special operators, hand-picked to fight humanity's oldest foe-the Kyrans.
Time and again they defeat the vicious saurians, knowing they'll get no credit for their victories. Hadrian and Mirra are separated by half a galaxy, by service, and by a lifetime's experiences. But when each of them is targeted in ruthless attacks, their hunt for answers sets them on a collision course. Their clash will decide the fate of an entire world-and reveal a shadowy new enemy that humanity and its allies are not prepared to fight.