Poul Anderson 
Rebel Worlds 
A Flandry Book

The barbarians in their long ships waiting at the edge of the Galaxy waited for the ancient Terran Empire to fall, while two struggled to save it: ex-Admiral Mc Cormac, forced to rebel against a corrupt Emperor, and Starship Commander Flandry, the brilliant young officer who served the Imperium even as he scorned it.Trapped between them was the woman they both loved, but couldn’t share: the beautiful Kathryn – whose single word could decide the fate of a billion suns.