The peace settlement that ended the wars between Earth and the Kzinti, stripping the Kzinti of all armed vessels apart from police craft, as explained in The Slaver Weapon.
The Treaty of Sirius is the agreement that closed the series of wars fought between humanity and the Kzinti long before the 23rd century. The Kzinti lost every one of those wars, and the treaty left them almost wholly disarmed, allowed to keep only a few police vessels.
It is described in the Animated Series episode The Slaver Weapon. Spock, Sulu and Uhura are lured to an ice planet by a Kzinti crew led by Chuft-Captain, whose ship, the Traitor's Claw, is officially a stolen police craft so that the Kzinti government can deny any part in what its crew does.
Chuft-Captain hopes that a weapon left by the ancient Slaver empire will let his people throw off the treaty's restrictions and return to conquest. The weapon destroys him and his crew instead.