The Battle of Installation 05 was an engagement between the Covenant forces, including those that fled the Battle for Earth, High Charity, and the Second Fleet of Homogeneous Clarity, and the UNSC-allied forces, including the UNSC In Amber Clad led by Miranda Keyes, and the Fleet of Retribution led by the Arbiter Thel 'Vadam.
AboutThe last years of the war saw a renewed push by the Covenant into human-occupied space, the discovery of unimaginable Forerunner constructs, and the destruction of the Covenant from within its own ranks.
Late War (2550 - 2552)
By 2552, humanity's death toll was in the billions. The mass construction of warships, combined with superior tactics, meant the UNSC was able to muster a formidable defense at any colony world left standing. In July 2552, a defense force of nearly fifty warships managed to hold off a Covenant assault of twenty-four ships at Sigma Octanus IV. Winning two-to-one odds was an impressive feat, and the fleet commanders were rightly commended.
Not a month later, the Covenant discovered and invaded the largest inner colony world, Reach. The UNSC threw every military asset at the planet to help protect it. Despite the heroic actions of the men and women of the UNSC, however, the Covenant overwhelmed the colony world with over 300 warships and countless ground troops. Aside from losing one of the major military strongholds humanity had, the battle claimed over 130 UNSC warships and around 700 million lives.
In the aftermath of the Fall of Reach, nearly all remaining UNSC forces were recalled to the defense of Earth. Even so, when the Covenant finally arrived in the Sol system on October 20th, 2552, the UNSC struggled to put up a formidable defense. Through the actions of Spartan John-117, however, humanity was managing to keep up the fight weeks into the battle. Large swaths of the planet were damaged or destroyed, with billions of residence fleeing off-world to try and survive humanity's last stand.
On November 3rd, 2552, the tide of the battle suddenly turned when one of the Covenant leaders, the Prophet of Truth went forward with his plan to replace the Sangheili with the Jiralhanae as the commanders of the military. This sparked a civil war between the Sangheili and the Jiralhanae, known as the Great Schism. With the Covenant forces suddenly turning their weapons on each other, humanity was able to reclaim the upper hand and begin to push Covenant forces off-planet.
The Prophet of Truth made a last-ditch effort to win by using the Portal at Voi to travel to the Ark and activate the Halo Array throughout the galaxy, killing all sentient life. Spartan John-117, along with Arbiter Thel 'Vadam, Commander Miranda Keyes, and Sergeant Avery Johnson followed the Prophet's remaining forces and stopped the Prophet, killing him in the process. The fighting was finally over. Humanity hadn't won so much as the Covenant had lost. But the war was over nonetheless.