Tiger by the Tail by Poul William Anderson (1926 - 2001)

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