Battle of Installation 04 (Earth-2042)

The Battle of Installation 04, also known as the Halo Event and the Halo Conflict, refers to a series of skirmishes between the Galactic Republic and the Confederacy of Independent Systems on the surface of Halo Installation 04, a Forerunner megastructure orbiting the gas giant Threshold. The Flood and the Forerunner Sentinels joined the conflict in its later stages. The battle became the turning point of the Human-Covenant War and ultimately led to the Great Schism.

Source

 * https://www.halopedia.org/Battle_of_Installation_04