In 2183, a rogue Spectre named Saren Arterius attacked the Citadel through a functioning mass relay hidden inside a statue in the Presidium. Saren brought a combined force of krogan and geth infantry from Ilos, timing the attack so that he would be at the Citadel's controls when the geth fleet and his flagship, Sovereign, arrived. In the battle that followed, the Destiny Ascension dreadnought evacuated the Citadel Council but ordered the wards' arms closed, sealing the geth inside an impregnable shell, cutting off any reinforcements or escape. This combative tactic would have wholly destroyed the Council fleet were it not for the actions of Commander Shepherd, who had followed Saren through the relay from Ilos.
Led by Shephard's ship Normandy, the Systems Alliance's Fifth Fleet assembled under the command of Admiral Hackett and waited just outside the Citadel's closed arms. Meanwhile, Shepherd fought Saren's forces inside the Citadel and eventually forces the ward arms open again. The Normandy defended the Destiny Ascension as it fled, saving the lives of the Citadel Council. Taking its cue from the Normandy, the Fifth Fleet supported the withdrawal, and took heavy losses from Sovereign's advanced firepower. At about the same time Shepherd killed Saren, Sovereign's once-impervious kinetic barrier overloaded and the Fifth Fleet focused its military might to shatter the flagship. The geth fleet was soon decimated without its leader.
In gratitude for the many human lives sacrificed to save it the Council made the unprecedented move of offering humans the chance to become a Council race. Ambassador Donnel Udina and Captain David Anderson accepted this honor on behalf of their species.