The Battle of Sigma Octanus IV was a large-scale space and ground battle between UNSC and Covenant forces in the Sigma Octanus system in July 2552. The battle is notable for several events, including the Keyes Loop, a maneuver performed by the UNSC Iroquois that resulted in the destruction of one Covenant destroyer and two Covenant frigates despite the Iroquois being outmatched four ships to one. A subsequent space battle involving 48 UNSC and 24 Covenant warships resulted in a UNSC victory, largely due to a daring move by the UNSC Cradle, a support vessel, which got between the two fleets and absorbed the opening Covenant volley.
Despite the overall victory, the UNSC and civilians on the surface still suffered heavy casualties, including the loss of 25 warships and 300,000 civilians killed. In addition, the Covenant were able to recover what they came to the system for in the first place, a Forerunner crystal containing the location of Installation 04. The Covenant were also able to track the UNSC Iroquois after the battle, allowing them to locate Reach, which soon after led to its destruction.