Stardate 44000.4:  The Federation starship USS Enterprise, under the command of Captain William T. Riker, failed to recover Captain Picard and destroy the Borg cube.  Instead, the cube assimilated Earth, turning Sector 001 into another Borg colony world.

With the seat of the Federation now in ruin, the rest of the Federation members moved quickly, calling out to every neighboring race, asking for assistance.  The next target on the Borg's agenda was Vulcan, and while Starfleet did it's best to fend off the incoming cube, it was the Klingon High Council which acted to save the lives of over a billion Vulcan citizens, by evacuating them from their homes.

After the assimilation of Vulcan, it was clear that each of the Federation member worlds could easily be their next target.  It was then that K'mpec came forward with the might of the Klingon Empire and took the reins of the remaining mobile forces, including Starfleet (what was left of it).  The surviving members of the makeshift Federation Council voted unanimously to accept the Empire's leadership, by forming a new body of government: the Galaxy Alliance.