The Imperialists were a militaristic faction of human origin which essentially took control of the human government in 15500 C.E.Y. through a system-wide coup. They quickly expanded their power through aggressive negotiations with the Phage weapon, creating the Imperial Alliance.
The Alliance was used to wage war upon the one defiant sector of the galaxy, against the Order of the Fist. The Imperialists eventually panicked, fearing that the stronger Order would come to destroy them, and unleashed the Phage upon their home system in 15604 C.E.Y. This led to the rapid spread of the organism throughout the galaxy, and the near destruction of sentient life.