On Sigma Octanus IV, the Covenant were defeated. On the ground, the Spartan-IIs secured a Forerunner artifact covered in symbols that the Covenant tried to transport out of Côte d'Azur. However, in space, the Covenant placed a Spy probe on UNSC Iroquois, which led the Covenant to the UNSC's stronghold and main headquarters, Reach.
The Fall of Reach resulted in staggering losses for both sides, but ended in a Pyrrhic Covenant victory. UNSC Pillar of Autumn was among the surviving UNSC ships which fled the battle, and made a unique jump, as described by AI Cortana as using symbols on the retrieved artifact from Sigma Octanus IV, were in fact very old star charts. The coordinates led to the Forerunner ring world of Halo.