Tiger by the Tail - Poul William Anderson

The haughty, horned aliens from the planet Scotha had very well organized intentions of conquering the Terran Empire—and Captain Dominic Flandry, Terra's ace saboteur, suddenly found himself in a strategic position to louse up the works. How? Well, Achilles had a heel ... and what else could you call a Scothani? A great Poul Anderson story! - Summary by author