Battle of the Maginot Sphere

"Something is wrong! It's moving away! At night I can see it—flitting shadows—black against the stars. Thousands of ships! Not spiraling outward, but heading for the line! This is the tipping point, Didact. It's no longer feeding. It's coming for you."

- The Librarian to the IsoDidact.

The Battle of the Maginot Sphere was the final naval engagement during the Forerunner-Flood war. Following the revival of Mendicant Bias by the Flood Graveminds, the Flood were gathered together and ceased feeding and expanding. The sole direction of Flood efforts was dedicated to piercing the Maginot Line and reaching the lesser Ark in order to stop the IsoDidact from firing the Halo Array and thus shattering the Forerunners' last hope of defense. To delay Mendicant Bias long enough to allow the Halos to fire, the IsoDidact deployed the AI's replacement, Offensive Bias, to meet the latter AI at the Line.

It should be noted that there were several battles fought at the Maginot Sphere; this particular instance refers to the last one, and the only one for which intact records survive. This final battle took place shortly after the annihilation of most surviving Forerunners at the Battle of the greater Ark.

Mendicant Bias
The battle was preceded by the rampancy of the Forerunner artificial intelligence Mendicant Bias, who, after a protracted intelligent conversation with the Flood, had decided that the Flood were the next step of evolution for galactic life, and that the Forerunners were corrupt for resisting this inevitable evolution. For refusing to be supplanted by their biologically superior adversary, Mendicant declared its allegiance with the Flood and defamed the Forerunners, not only lending its tactical intelligence to the Flood by directing their fleets, but also gave the thousands of Forerunner-crewed vessels under its command to the Flood. However, its intelligence was tinged with madness, a blind fury for the Forerunners and for their obstinate tendencies, making its actions predictable and violent, and thus easily countered by the Forerunner's last great AI: Offensive Bias, designed along the same lines as Mendicant, seemingly possessing less free will and creativity, but being exponentially more lethal.

Invasion
Shortly after Mendicant Bias assumed command over the Flood forces, he began ordering termination of the Flood's feeding and exponential growth, and instead, Flood forces were marshaled to penetrate the Maginot Sphere, the Forerunner final line of defense for their inner colonies. If the Maginot Sphere was penetrated, the Forerunners would be doomed, thus making this attack an influential battle that would decide the course of the war. Whereas previously captured Forerunner superluminal ships were sequestered and unused in the Flood war effort, Mendicant Bias now recalled all of them to attack the Maginot Sphere, and the gathered Flood attack fleet was over 4.8 million vessels, although the majority were non-military, and an extremely small number were capital-scale warships. The fleet was led by Mendicant's core battlegroup of 1,000 Forerunner warships.

Destruction of the greater Ark
The last Forerunner bastion was located at the greater Ark and Omega Halo. Despite having possibly the greatest remaining concentration of Forerunner might, Mendicant Bias, by utilizing Flood-controlled Precursor star roads and its millions of vessels, was able to overwhelm and annihilate the last major refuge of the Forerunners. Despite its numerical superiority, Mendicant was unable to prevent the escape of a small number of warships, but most dangerously failed to capture or kill either the IsoDidact or Offensive Bias.

The Flood arrive and Halo is armed
"Ironic that in this deadly act there is such grace and perfection of execution. This will be the greatest combined operation in the history of Warriors, in the history of Forerunners. It is proceeding flawlessly."

- The IsoDidact, as Offensive Bias is deployed to stop Mendicant.

Recognizing that it was no longer possible to win a conventional victory, the IsoDidact ordered the Halo Array to be primed and fired. However, Mendicant Bias had located the portal that led to the lesser Ark. Fortunately, Offensive Bias arrived with what little it had managed to salvage from the fall of the greater Ark.

The IsoDidact was able to marshal the Suppression, Security, and Emergency Circumstance fleets and placed them under the command of Offensive Bias. As the IsoDidact and Offensive Bias primed the Array, the Flood began to arrive. Mendicant Bias immediately sent a coded message to Offensive Bias stating that it would give no quarter and would destroy the Ark. Mendicant then offered Offensive a chance to join it and survive; Offensive informed the IsoDidact that it had rejected the offer. Once the Ark was armed, Offensive Bias moved its fleet to the Maginot Line to block and delay Mendicant Bias, who was approaching the portal to the Ark: with a total of over eleven thousand vessels, Flood outnumbered the Forerunners 436.6:1. However, Mendicant Bias's forces did not include the near-indestructible star roads or any other Precursor artifacts which had allowed the Flood to dominate the Forerunners in earlier engagements.

Flood assault
The point where Mendicant Bias and its fleet breached the Maginot Sphere was predicted by Offensive Bias beforehand, allowing the latter to more efficiently prepare for and counter the incursion. Two hours after the Flood fleet appeared Mendicant Bias made the first move, devoting over a quarter of his vessels — over 1.7 million small and lightweight leisure craft — to storm the Forerunner fleets, not to attack them with weapons, but rather, to overwhelm them, providing too many targets for too few weapons, and boarding Forerunner ships to infect and capture them. Remarkably, the Forerunner vessels were able to evade boarding for three and a half hours. It was only after this period that larger commercial vessels and smaller military craft were launched by the Flood, and the first Forerunner ship was boarded, although it suicidally rammed an incoming Flood ship, releasing over thirty thousand Flood warriors, killing them as the Forerunner crew perished. For four hours, the Flood attack waves intensified, culminating in a final wave of container and transport vessels and warships, most far heavier than a dreadnought. Despite the fact that Offensive Bias could have easily countered the earlier moves, it chose to let its ships act as bait, a seemingly incompetent decision; it was intentionally fighting "just well enough to seem lucky", playing into Mendicant's contempt for the Forerunners and Offensive Bias alike. Regardless, it was becoming clear that Offensive would soon run out of options, regardless of its opponent's ability.

Halo is fired
"It is best that our crews perished now; because the battle that is about to ensue would have driven them mad."

- Offensive Bias

However, the entire battle up to this point had been a feint — Offensive Bias had merely been playing for time and destroying Mendicant's core vessels and allowing its ships to be captured to be utilized later. As the battle intensified, the Halo Array fired, instantaneously killing all Forerunner and Flood in its area of effect and leaving the majority of the ships lifeless and drifting. The odds were suddenly inverted: whereas the Flood had outnumbered the Forerunners by millions, it was now Offensive Bias that had more vessels than Mendicant, a result of it having had full control over its fleet. In addition, with the crews dead, Offensive Bias no longer needed to concern itself with their safety and promptly exploited that fact.

It could now accelerate the vessels under its control well beyond velocities safe for biological beings, or open uncontrolled slipspace ruptures across the battlefield, distorting the very fabric of reality around them. Captured Forerunner vessels began to self-destruct or opened uncontrolled slipspace rifts that damaged many former Flood vessels. The sheer number of drifting derelict ships began to hinder Mendicant's core fleet — they began to collide into the ruptured fuel cells of the drifting vessels, destroying fifty-two of Mendicant's warships in only a few seconds when it attempted to use them as cover.

The final portion of the battle was fought in the upper atmosphere of an unspecified world. Over the next few minutes, over five hundred more of Mendicant's warships were destroyed by collision, the uncontrolled Slipspace rifts, weapons used at point-blank or even contact range, and so on. The unorthodox, unprecedented tactics worked brilliantly — within ninety seconds, the Forerunner ships controlled by Offensive Bias outnumbered Mendicant's surviving fleet six-to-one.

Conclusion and aftermath
Three minutes later, the eight-hour battle was over. Most of the remaining Flood ships had been reduced to debris, and only one of Mendicant's former Forerunner warships that was part of his original thousand-strong core fleet remained. Offensive Bias retrieved Mendicant Bias's personality construct array and sent it to Installation 00 for research. On the Ark, the IsoDidact placed Mendicant Bias on trial for its betrayal and subsequently sentenced the AI to indefinite stasis, its processes locked in a state of penitence.

However, a Keyship with a remaining active fragment of Mendicant Bias eventually made its way to the San 'Shyuum homeworld and used both in battle during the Sangheili-San 'Shyuum war and later as a power source for their world ship and holy city High Charity. When the Keyship arrived at the Ark 100,000 years later, Mendicant's primary incarnation on the Ark was able to use the missing shard within to reconstruct and reactivate itself, subsequently seeking atonement by helping John-117.

Trivia
"Maginot Line" is the translated name for the Forerunner defensive boundary known in their language as the "Jat-Krula"; a result of the the automated translator's usage of terminology familiar to the reader to provide context. The Maginot Line was a French defense, which ran along the Franco-German border to defend against the invading Nazis during World War II. Ultimately, it failed due to being rounded by the German invasion through Belgium and the Netherlands, while a decoy force sat opposite the Line in Germany, deceiving the French into believing that the Germans would attack across that border.

List of appearances

 * Halo 3
 * Terminals
 * Halo: Silentium