Biological Weapons Solve Everything: Once the terraforming process on SecLand was complete and the New Pallas Republic and the Society for the Preservation of Parahuman Species lost their last reason to cooperate (and the arcologies were unpressurized), both sides deployed biological weapons against one another. The SPPS deployed a generalized plague designed to kill as many as possible, with a slight inclination to be more deadly to the scions of Argentum. New Pallas, on the other hand, took advantage of the cloning employed by the SPPS to tailor their plague to specifically wipe out the priestly and military castes. While both states had their ruling classes decimated, New Pallas had enough survivors, and social mechanisms for recruiting new leaders from the “rabble” to rebuild while the SPPS was completely decapitated and vulnerable.
The genetic bottleneck generated by the plagues are also why the modern High Families (those who most closely resemble their progenitor) of genus Argentum are so inbred.