Born and bred to hunt, they had never encountered a species thay couldn't treat as prey - until they met the canny pseudo-pacifists from Planet Earth. They nearly overwhelmed humanity on first contact, but fast as you can say "Genghis Khan" or "Alexander the Great" the seemingly harmless monkey boys were all over the pusseycats like ugly on an ape, with space fleets and strategic thinking that left the Warrior Race quite dazzled.
But that was then and this now.
The pain of lost battles has faded and the Kzinti are back, spoiling for a fight. Larry Niven's Known Space is again aflame with war.