The Battle of Installation 00 was a major conflict between the forces stationed aboard the UNSC Forward Unto Dawn and Arbiter Thel 'Vadam's Fleet of Retribution, and the Covenant fleet loyal to the Prophet of Truth. The fighting was the last battle of the Human-Covenant War and culminated in a UNSC victory.
Once the Prophet of Truth and his fleet used the Portal at Voi to leave Earth and head towards the Ark, Miranda Keyes, now commander of the UNSC Forward Unto Dawn, along with the Fleet of Retribution, got permission to pursue Truth in an effort to stop the firing of the Halo array. Upon arrival, the Arbiter's fleet determined they were greatly outnumbered, but were still ultimately victorious over the more numerous but significantly less experienced Jiralhanae-commanded ships. With Truth's fleet destroyed, only Truth and those loyal to him on the Forerunner Dreadnought remained. A race to the Citadel ensued, and Master Chief, along with the rest of the Forward Unto Dawn forces, were able to kill the Prophet of Truth prior to the Ark's activation, though at the cost of Miranda Keyes' life.
The situation was complicated by the presence of the Flood, who arrived via High Charity at the same time, and also wanted to stop the firing of the Halo array. Once the threat of Truth was eliminated, however, the Flood forces immediately turned on John-117 and the rest of the UNSC forces. It was determined that the only way to stop the Flood was to recover Cortana from High Charity and fire Installation 08, the replacement Halo for the recently destroyed Installation 04. This plan was eventually successful, although sergeant Avery Johnson was killed during the conflict.
With Cortana recovered and Installation 08 preparing to fire, the remaining UNSC and Arbiter-loyalist forces evacuated to the Forward Unto Dawn. Master Chief, the Arbiter, and a few others were successful in reaching the ship, and escaped into the portal connecting the Ark to Earth just as Installation 08 fired. While half of the ship, including the portion containing the Arbiter and all other remaining UNSC personnel, made it back to Earth, the other half which contained only the Master Chief, was set adrift in space.