Battle of the Quarantine Zone

"We shall cut into the heart of this infestation, retrieve the Icon, and burn any Flood that stand in our way!"

- Rtas 'Vadumee

The Battle of the Quarantine Zone was a battle which took place on Installation 05 for a span of nearly 100,000 years. When the ring was later discovered in 2552, the Covenant and UNSC fought through the Quarantine Zone in search of the Activation Index, known to the Covenant as the Sacred Icon. Both parties had to battle their way through legions of Flood and Forerunner machines designed to halt the parasitic outbreak.

Background
Not much is known of how the battle of the Quarantine Zone began, as it is rooted in ancient history. The battle began after an unknown event, approximately 3,000 years after the activation of the Halo Array, when the Flood under containment on Installation 05 were released. Under normal circumstances Sentinel guards would have been able to keep the parasite under control. Unfortunately, the Flood had access to hosts, and were able to hold off the constructs.

After building enough biomass, a local Proto-Gravemind reached critical mass, allowing the formation of a sentient, highly aware form of flood stage known as the Gravemind. Now coordinated, the Flood posed a serious threat. The battle soon spiraled out of control, and Sentinel Constructor Factories were built to construct more powerful constructs, such as Sentinel Majors and Enforcers. An enormous wall was built around the Installation's Library, which was housing the Gravemind. The area within the boundaries of the wall were starved of heat and light in order to inhibit Flood growth, and thus the Quarantine Zone was established.

Over the centuries, the Gravemind's reach spread throughout the Installation. The Monitor of the installation, 2401 Penitent Tangent was captured by the Gravemind, giving him access to the teleportation grid, and causing the structures on the Installation to fall into disrepair.

The human mission
Inadvertently arriving at Installation 05 after following a Covenant ship, one of Commander Miranda Keyes' primary goals was the capture of the Forerunner activation key: the Index. To know exactly what she was up against, she accessed all information (classified or not) to do with the rings from AI Cortana.

While SPARTAN John-117 was busy on a special mission to assassinate the High Prophet of Regret, some of the In Amber Clad's Marine and ODST contingent; along with their Pelicans, were called back to take part in the mission.

Breaching the outer wall with ease, the 100+ Marines led by Keyes and Sergeant Major Johnson then entered the second wall. The shield down temporarily, the UNSC In Amber Clad was brought into the Quarantine Zone to provide support. The UNSC was ill-prepared against the Flood, the Marines' numbers began to shrink, many of whom were not even briefed on their existence.

Escaping the wall defences, the remaining Marines were welcomed by the arrival of reinforcements of the vehicular kind. Although armed with the Clad's remaining supply of tanks and warthogs, UNSC superiority was short-lived. Reaching a large-sized anti-gravity gondola designed to take them to the Library interior, there were only a few Marines accompanying Keyes and Johnson.

By the time they began their journey, the Flood had not only captured Pelicans, but they had already begun their assault on the Clad. While the ground troops fought in the structure, the ship was lost to the parasite. Now with an FTL-capable vessel in their hands, the Flood's hundred-thousand year battle was over.

Covenant intervention
Thel 'Vadamee, the Arbiter of the Covenant, was personally ordered by the prophets to journey to the surface of the ring to retrieve the Index (or the Sacred Icon as the Covenant call it), some time after the UNSC began their operation. Dropped directly into the outer wall by a Phantom transport, the Arbiter fought mostly alone, only aided by a small number of the original landing party (several Unggoy and Kig-yar), through numerous Forerunner constructs to reach the control center for the containment shields. By lowering these shields, the Covenant soldiers learned that human Marines had made it through the wall first, only to be swiftly infected by the Flood. By lowering the Containment shields, the Covenant allowed more free movement for the Flood. More Covenant warriors soon entered the Quarantine Zone along with their own vehicular support, unfortunately exacerbating the Flood outbreak.

This breach of containment led to the rapid growth in the Flood's numbers as they assimilated both humans from the and Covenant warriors. With these new hosts, the Flood gained access to more sophisticated equipment, weapons and combat vehicles, magnifying the danger the outbreak presented immeasurably. The Flood showed a bit of a tactical genius by destroying one of the Forerunner Sentinel manufacturing plants, bringing it crashing down on the surface and stopping the production of Sentinels and Enforcers. This meant that while the Flood might not have to fight as many Forerunner machines, human and Covenant forces would possibly be able to get to the Index easier.

Despite this new and surprising defense, Covenant and human forces were able to battle their way through the legions of Flood and Sentinel constructs. Elements of both parties eventually reached the gondola entrance points to the Library of Delta Halo. Commander Miranda Keyes, Sgt. Johnson and a number of Marines entered the Library first. They were quickly followed by the Arbiter and a squad of Sangheili SpecOps troops. Rtas 'Vadumee stayed behind to defend the Gondola station.

The Library
Human forces had gained access to the Library first, but only by a matter of minutes. Upon arrival within the Library it was clear that the humans had faced stiff opposition, all but two of their forces had died combating the Flood and Sentinels already within its walls.

The Arbiter found Miranda Keyes and Sergeant Johnson within the Index Chamber, having retrieved the Index seconds before his arrival. Sergeant Johnson was ready, armed with a Battle Rifle, along with Miranda Keyes who was reaching for the Index holding on to one of the Gravemind's tentacles. Miranda secured the Index and drew two SMGs. Johnson called for his marines, but received no answer and armed his weapon. The Arbiter proved his worth as a warrior once again, incapacitating both Johnson and Keyes in a matter of moments. The arrival of Tartarus within the Chamber signified the beginning of the Great Schism as he attacked the Arbiter, sending him plummeting to his doom deep within the Library, towards the Gravemind.

Conclusion
The battle itself was from a strategic stand point, a Covenant victory. They succeeded in not only obtaining the Index but captured all of the Marines that were not infected (the Covenant were well aware that to activate the rings a human hand was required). The Flood on the other hand, won a tactical victory, succeeding in not only escaping Installation 05 but gained the means to spread to other worlds through infesting all the nearby ships (i.e. In Amber Clad). The UNSC forces, on the other hand, failed to recover the Index and were almost completely killed or captured by the Covenant or infected by the Flood and the Forerunner constructs, without the leadership of 2401 Penitent Tangent could not stop the Flood from escaping the ring.

Possible human-Sentinel alliance
There is supporting evidence to entail and suggest that the Marines were never attacked by Sentinels but were in fact assisted by them. This is not out of the question because, like on Installation 04, the Sentinels might have identified the Marines as Reclaimers, rather than how Sentinels branded the humans as an "Aboriginal Subspecies" on Onyx. This would also explain their open hostility to the Covenant who were enemies of the Humans.

For starters, the Arbiter encounters an enormous and massive number of human combat forms inside the Sentinel Wall immediately after he deactivates the shield. This suggests that the humans had been deployed much earlier which should have been impossible. First of all, the marines should not have been able to deploy immediately after the barrier went down because In Amber Clad would've first had to slip past the Covenant forces that were moving in to secure the inner wall. However, if the Sentinels had chosen to assist the marines, they would've deactivated a small portion of the barrier to let them in. Also, in the area where the UNSC Pelican flies over head, there are a number of Sentinels and even an Enforcer present. None of which seem to show any hostility towards the Pelican. Another factor is simply that considering how much trouble Covenant Elites had getting inside the Library, it doesn't seem likely that the marines would've been able to get past the Flood and the Sentinels. However, if the Sentinels had been helping them, then it is much more likely that they would have succeeded.