Zero Day is the codename for an event that occurred 22 years ago where a US military outpost guarding an unknown alien device in an undisclosed location of the USA suddenly came under attack upon the device’s activation.
The device was a Kryptonian warpgate that sent an invading fleet of ships and androids to take down the soldiers of the outpost with its leader, codenamed Nemesis Omega, slaughtering the ground forces. The attack only ceased when a blinding flash of light appeared behind Nemesis Omega and caused the warp gate to deactivate, causing the forces already on Earth to shut down and trapped Nemesis Omega wherever he was behind the portal.
While the casualties were high, Amanda Waller and Sam Lane both managed to survive at the last second from the robots shutting down and they confiscated the alien tech to research, reverse engineer and prepare for another alien invasion in the future.