Following the fall of Earth in 2134, the Ur-Quan opted to leave a larger than usual garrison to enforce the Oath of Fealty because of the Human's aptitude for tenacity and cunning, the central role they had played in the Alliance of Free Stars, as well as their homeworld's abundance in natural resources and industrial capacity. The Ilwrath and Spathi were the nearest Battle Thralls to Human space.
When the Kzer-Za disappeared to fight the Doctrinal Conflict, discipline broke down. The Ilwrath left to join in the hunting of the Pkunk, and their remaining Spathi subordinates soon let their own paranoia get the better of them. They accordingly made a series of "strategic redeployments" by moving the entire group to Mars, then to Ganymede, later to Titan, and finally to Pluto. By rigging the Hierarchy Moon Base to appear active, the Spathi effectively kept the Humans from learning about this migration. Earthguard was further whittled down as Spathi ships one by one returned home.
The defeat/defection of Earthguard liberates Human-space just enough for the fledgling Alliance to begin operations.