With the end of the war with the Covenant came an awakening of several Forerunner installations that had remained dormant since the time of the firing of the Halo Array 100,000 years before. Among those awakened facilities was the Ark, whose monitor 000 Tragic Solitude held a grudge against humanity for the destruction caused on the installation's surface. This led to a series of events culminating in an attempted invasion of Earth, which was halted through the combined actions of the UNSC and Swords of Sanghelios.
Conflicts with Forerunner creations were left mostly to localized incidents until the discovery of Forerunner Shield World 001, or Requiem, in 2557. While both the UNSC and former Covenant under command of Jul 'Mdama battled in orbit and on the interior surface of the artificial world, the Ur-Didact, a Forerunner in stasis in a cryptum for the past 100,000 years, was awoken and attempted to complete his goal of wiping out any and all remaining humans from the galaxy. Though his plan was not successful, he did still manage to kill seven million people from the city of New Phoenix before he was stopped.