The Sanctrian Takeover is the result of the nuclear attack agenst the Sanctrians who where working on the Slip-space drive.
the Sanctrian Takeover was lead by the leaders of Sanctrist City, this massive military movement targeted mainly resistant military, nuclear weapons,and governments. the Sanctrian Takeover succeeded because the human species at the time was at tier 8 and the Sanctrians where tier 6.3(as they were trying to reverse engineer the Arcam Vault Ship). Although the Sanctrians succeeded on their takeover it wasn't as clean as the Sanctrians have hoped for, some countries were completely obliterated because of the insane amout of nuclear devices that they used to self-destruct rather letting their people to remain unharmed.
after this event the Sanctrians continued their research of the Slip-space drive in which they succeeded and gain attraction of the Coalition.