Tiger by the Tail by Poul Anderson

Captain Dominic Flandry wakes drugged, stripped, and helpless aboard a barbarian warship far beyond the Empire's reach. His captors are no mere raiders, but the Scothani: a fierce, technologically capable people whose hidden strength, rigid discipline, and expanding ambitions could ignite the long-feared collapse of imperial order. Dragged before the proud prince Cerdic and the hard-eyed King Penda, Flandry is threatened with torture, forced to study their language, and warned to tell the truth or face telepathic interrogation. Yet the Imperial intelligence officer survives by doing what he does best—reading people, disarming enemies with charm, and turning apparent weakness into leverage. As he wins back his possessions, gambles with his captors, and probes the tensions of Scotha's brutal court, he uncovers a semi-feudal civilization armed with advanced weapons and ruled by rival nobles, fanatics, and opportunists. Flandry soon realizes the Scothani are not just a border menace, but a serious galactic threat, and that his own survival may depend on exploiting their fractures before they can strike the Empire. This edition has been fully restored with modern typesetting, custom cover design.