The Man-Kzin wars
It was so unfair--here the Kzin were, warcats supreme, bringing the galaxy piece by piece under feline dominion, carving out satrapies for the home planet like the lords of creation that they were--and then they ran into those pesky humans...and the Kzin had lost their first war ever in centuries of conquest. Still, you can't keep a good warcat down, and the Kzin have by no means given up. New weapons, new strategies, and new leaders--the humans had better keep their powder dry.