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."

- Librarian to the Didact

The Battle of Maginot Sphere was a naval battle in the Forerunner-Flood War. It marked a turning point in the war — the Flood had stopped feeding and expanding, and instead, were devoting their efforts to penetrating the last Forerunner line of defense, the Maginot Sphere, to complete the destruction of the Forerunners.

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 and for refusing to be supplanted by their biologically superior adversary. Mendicant declared his allegiance with the Flood and defamed the Forerunners, and not only lent his tactical intelligence to the Flood, directing their fleets, but also gave the thousands of Forerunner-crewed vessels under his command to the Flood. However, his intelligence was tinged with madness, against blind fury for the Forerunners for their obstinate tendencies, making his actions predictable and thus easily countered.

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 destined for death, thus making this attack an influential battle that would decide the course of the war. Whereas previously, captured Forerunner super luminary ships were sequestered and unused in the Flood war effort, now, Mendicant Bias 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 Mendicants original 1000 Forerunner warships. The Didact, who was able to predict where the Flood blow would fall upon the sphere, was able to marshal the Suppression, Security, and Emergency Circumstance fleets under Mendicants "brother", Offensive Bias, with a total of over eleven thousand vessels, with the Flood outnumbering the Forerunners 436.6:1.

Flood Assault
Two hours after the Flood fleet appeared, Mendicant Bias had 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 infest and capture them. Remarkably, the Forerunner vessels were able to evade boarding for three and a half hours, and, despite having too few weapons to target the leisure craft, remained unboarded. It was after three and a half hours, 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 Forerunner Dreadnought.

Firing Halo
However, the entire battle up to this point had been a feint — Offensive Bias had merely been playing time and destroying Mendicants core vessels. 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, drifting, un-piloted. The odds were suddenly rebalanced; whereas the Flood had outnumbered the Forerunners initially by hundreds and hundreds, now, it was the Forerunners that had more vessels than Mendicant. Captured Forerunner vessels began self-destruction or opening uncontained Slipspace rifts that damaged many Flood vessels, and the sheer number of drifting derelict ships began to hinder Mendicants core fleet — they began to collide into the ruptured fuel cells of the drifting vessels, destroying fifty-two of Mendicants warships in only a few seconds. Over the next few minutes, over five hundred more of Mendicants warships were destroyed by collision, the exposed Slipspace rifts, and so on. The battle turned even further — the Forerunner battleships, all controlled by Offensive Bias, now outnumbered Mendicant's fleet six-to-one.

Conclusion
Three minutes later, it was all over. Most of the remaining Flood ships fled had been reduced to debris, but still one of Mendicants 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, his mission complete. However, he did not realize that on board one of the dreadnoughts, a fragment of Mendicant Bias remained. This dreadnought was eventually located by the San 'Shyuum and used both in battle during the Prophet-Elite War and later as a power source for their world ship and holy city High Charity.

Trivia

 * The Maginot Line was a French defense ,which ran through Belgium downwards, against the invading Nazis during World War II. Ultimately, it failed due to a gap in the defense, and France fell.