Okay, [b]major[/b] spoilers for the Forerunner books and Halo 4 ahead. [b]Human-Flood War[/b] I'll start from the origin of the Flood. They were first encountered by the humans, and came in the form of a fine powder contained in cylinders. The humans introduced this powder to a pet called the Pheru, which caused them to have qualities that the humans liked. San Shyuum also took the mutated Pheru as pets. Later, the Pheru continued to mutate and had growths such as rods on their heads. Eventually they became the Flood. Before this, the humans had formed an alliance with the San Shyuum on Charum Hakkor. Note that the humans' technological prowess rivaled that of the Forerunners. The humans went to war with the Flood. In their desperation, they invaded many worlds. And the humans found a cure to fight back the Flood. They were able to eradicate the Flood, so that it would not be a threat until thousands of years later. [b]Human-Forerunner War[/b] The Forerunners saw human invasion of planets as expansion, not knowing that they were trying to escape from the Flood. As upholders of the Mantle, the Warrior-Servants among the Forerunners, led by the Didact, waged war against humanity. Humanity(and the San Shyuum) was forced to fight a war on two fronts, and naturally lost. Naturally, the humans did not give the cure to the Flood infestation to the Forerunners, destroying all data of it. As a final act of punishment for humanity's apparent crimes, the Forerunners(and the Didact) devolved the humans to a pre-technological state and sent them back to their homeworld Erde-Tyrene(Earth). They were overseen by the Librarian, hence the Didact referring to the humans as the Libarian's pets in Halo 4. [b]Forerunner-Flood War[/b] The Forerunners used tactics such as orbital bombardment to stall the advance of the Flood. A high-ranking Forerunner known as the Master Builder built the Halo rings as a last resort to destroy the Flood. The Forerunner Council favoured this option over the Didact's proposed Shield Worlds. Later, the Didact returned and the Master Builder was trialed for this was a major violation of the Mantle. The Didact led the Forerunners against the Flood, and eventually used the Composer to transform his warriors into Prometheans, which were used to combat the Flood. There weren't enough, so the Didact decided to Compose a whole lot of humans to make Prometheans. He justified this by saying that the humans had a noble purpose in becoming Prometheans as they would help stop the Flood and save the universe. In the end, all of the Forerunners' tactics failed as the Flood consumed entire systems. The Forerunners were on the verge of being overrun. As a last resort, the Halo Array was fired as a last resort, eradicating all sentient life - any potential Flood hosts. Without hosts, all Flood died out, except for those contained in Forerunner installations such as Installation 04(i.e. Halo CE Libary). Ships reintroduced species into their respective planets, allowing life to start again. [Edited on 11.20.2012 8:24 AM PST]