Forerunner-Flood war

"A hundred thousand years ago, a great civilization existed in this Universe. Like all great civilizations, they faced a sudden and dire turn of events. A threat to their primacy from outside. Something they never expected, never prepared for... a deeply alien threat they called simply The Flood."

- Cortana

The Forerunner-Flood war was a galactic conflict that occurred between the Forerunners and the Flood parasite. It was started on G617 g1 some 102,000 years prior to the present day, and lasted approximately three centuries.

Background
The Forerunners originally evolved in and colonized the Milky Way galaxy, organizing themselves over a large number of terrestrial planets and colonizing millions of worlds, as well as forming a structured military which included both naval elements and ground forces. The Forerunners were of sufficient technological capability to construct superluminal vessels, create sophisticated artificial intelligence and set up massive superweapon networks. They measured their advancements in Technological Achievement Tiers.

While powerful, the Forerunners were not the only major civilization in the galaxy. Other races had also developed interstellar empires, including an alliance between prehistoric spacefaring humanity and San 'Shyuum. Around 110,000 BCE, the Flood entered the galaxy from one of the Magellanic Clouds and caught the humans and San 'Shyuum by surprise, infecting their populations and taking their worlds at a steady rate. Eventually, humanity discovered a way to fight the Flood and drove them off the galaxy, but this secret was lost when the Forerunners dismantled human civilization in the aftermath of the human-Forerunner wars.

The threat of the Flood would shape Forerunner politics for millennia to come, with different rates having differing views on how to best prepare against their return. A series of devastating superweapons, known as the Halo Array, was created by the Builders while the Warrior-Servant Prometheans, led by the Didact, devised a more strategic defense solution consisting of numerous military installations known as Shield Worlds. However, the Prometheans lost to the Builders and were forced into exile, and twelve Halo installations were built.

Aware of the inevitability of the return of the Flood threat, the Lifeworkers led by the Librarian began indexing and protecting species across the galaxy, cataloging them at an extragalactic installation known as the Ark, as a method of countering the total elimination of all sentient species in the galaxy.

After over nine thousand years, the Flood returned, contesting the Forerunners for control of the entire galaxy. While the Flood sought to assimilate all sentient life, the Forerunners attempted to defend against the Flood threat using several measures to both directly combat them as well as carry out research in regards to their capabilities.

Beginning
"It devoured everything it touched. At first, their technology, their courage seemed like it might prevail. But they waited too long to see the threat, to join the fight. The Flood had spread too far and too wide."

- Cortana on the Forerunner-Flood war.

After ten thousand years of absence, the Flood reappeared on the planet G617 g1, a mostly barren planet harboring a small commune of Forerunners. The Flood initially caught the Forerunner military by surprise, using captured non-military vessels to penetrate local Forerunner naval blockades to descend and land upon Forerunner-colonized worlds, overrunning local defenses and converting billions of Forerunners per world with hundreds of millions of Flood forms within a few years. Eventually, Forerunner fleets were forced to commence orbital bombardment on Flood-infested worlds to prevent the Flood's spread to other planets.

Over a span of 300 years, the Flood took twelve Forerunner border systems. Until this point, the Forerunner leadership had downplayed the Flood threat, even keeping it hidden from the general populace. However, they were now forced to act in order to survive. During this stalemate, the Flood was exponentially growing and readying for an attack.

Tipping point
Around 100,043 BCE, a powerful Contender-class artificial intelligence, Mendicant Bias, was charged with the test firing of a Halo near Charum Hakkor. This test firing had an unprecedented side effect; it liberated the Timeless One, an entity that had been caged on Charum Hakkor for millions of years. After the entity was transported to the Halo installation for study, Mendicant Bias disappeared along with the ring.

Due to an unauthorized use of a Halo ring by the Master Builder against the San 'Shyuum by around 100,000 BCE, the Forerunner Council decided that the array be returned to the Forerunner capital and decommissioned; the weapons were deemed too devastating and a violation against the Mantle, the Forerunners' pledge to preserve all life in the galaxy. However, as the Halo rings lay in parking orbits over the capital, Mendicant Bias returned on the Halo ring that had been lost 43 years earlier. During this time, Mendicant Bias had become rampant, and turned against the Forerunners. The AI launched an assault on the capital, taking control of many of the Halo rings, while several Forerunner naval groups were recalled to defend the capital against the attack. The Halo rings not under control of Mendicant Bias saved themselves by entering the capital system's slipspace portal, returning to their place of construction, the Ark, while the Forerunner fleet assaulted the rings under the AI's control.

With the Forerunner government in disarray, the Didact, who had been reinstated as the commander of the Forerunner military, reactivated the Shield Worlds and the defense plan he had engineered thousands of years earlier.

Stalemate
The highest levels and tiers of the Forerunner Fleet Command began to realize that Forerunner species' extinction was plausible at the Flood's discretion as so many Forerunners had fallen victim to the Flood. After Flood spreading continued, planetary self-bombardment after Flood infestation turned into complete system-wide destruction by detonating planetary system stars after a large Flood presence was detected in a Forerunner system. Forerunner military forces were ordered to don heavy armor and other personnel were ordered into protective stasis. Flood were taken into M-type Forerunner installations and studied in an effort to find yet another countermeasure.

Hundreds of other unsuccessful methods were attempted by the Forerunners to overcome the Flood, and all possibilities were exhausted as a final countermeasure came into mind for the Forerunners: activation of the Halo Array and destruction of all sentient life in the galaxy, depriving the Flood of all biomass that they could consume, thus halting them. However, the Didact, the commander-in-chief of Forerunner military, adamantly refused to activate the Halos, saying that it would overturn the Mantle.

However, the Flood were growing in number, and were forming a centralized sentient intelligence to coordinate their efforts; Gravemind, and their raw computing power began to overrun the last Forerunner naval countermeasures, which consisted of using Keyship vessels and drawing the Flood into pricey naval engagements. However, even Forerunner commanders realized that their naval tactics were being overcome, and a new solution was necessary if the Forerunners were to pull of out this stalemate.

While there was the Maginot Sphere where the Forerunners held the Flood at bay, the Forerunner Librarian journeyed beyond this line at great personal risk, continuing her millennial mission of indexing and documenting sentient species across the galaxy, filling every vessel possible with documented species for transit to the Ark for safety in the campaign to save all species possible from the advancing Flood.

Endgame
"Something is wrong...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

During the stalemate, the Flood was exponentially growing and readying for an attack, and eventually they assaulted the Forerunner core systems with thousands of Flood-filled super luminary vessels. In response, three Forerunner naval groups, the Emergency Circumstance Fleet, Security Fleet, and Suppression Fleet, were recalled to defend against the Flood attack.

With naval strategies failing, the Forerunners turned to their Halo project as a final resort, distributing seven remaining installations across the galaxy. As they readied the array for firing, they secured other species into the Ark to preserve them from the array's pulse. Around this time, the Librarian journeyed to Earth and buried the the Ark portal there. The Forerunners saw a unique potential in the human species, protecting them with the Ark along with others but passing their Mantle down on them.

The Didact sent a rescue party for the Librarian, in an effort to send her to the Ark before he would be forced to activate the Halos and kill all sentient beings, but Mendicant Bias destroyed the rescue party, leaving the Librarian stranded on Earth away from the Ark. The collapse of the Forerunner military forced the Didact to activate the Halos. Mendicant Bias assembled all available vessels under its control and launched an assault on the Maginot Sphere to prevent the Didact from doing as so, but was stopped by a loyal Forerunner artificial intelligence, Offensive Bias.

While the Didact began the activation sequence for the Array, the Flood fleet attacked the remnants of the Forerunner fleet as it formed a spherical defensive formation. Using feint tactics, the Forerunner fleet held off the Flood as the Halos activated, annihilating all sentient life in the Milky Way galaxy. Those saved were sheltered on the Ark. The surviving Forerunners left the galaxy, the Flood were halted, and the centralized Gravemind form eventually died. Afterward, Keyships began escorting various alien life forms back to their homeworlds, so that life could finally return to normal, with no threat of the Flood.

List of appearances

 * Halo 2
 * Halo 3
 * Terminals
 * Halo Legends
 * Origins
 * The Forerunner Saga
 * Halo: Cryptum
 * Halo: Primordium
 * Halo: Silentium