system
Given the following post, respond as if you're commenting on Reddit:
human
[HALO] Why haven't the flood won again?

If the Forerunners, who made every single other galactic power in the galaxy look pathetic, couldn't beat them, how come a Tier 3 civilization has managed to keep them at bay? Is it because of the Forerunner's efforts?
gpt
It was because the flood had a lot of time to prepare against the Forerunners, and the Forerunners didn't take it seriously. It was actually ancient humans who discovered the flood, and they did a good job keeping it under control until it spread to a Forerunner planet. In order to stop it they just blew up the planet which caused the Forerunners to declare war on them in retaliation. Humans couldn't fight the flood and Forerunners so they lost and were sent back to the stone age (literally). The flood meanwhile was growing and even still the Forerunners refused to take it seriously because they basically thought the humans were just exaggerating the whole thing. 

Eventually they realized the flood was a real threat but by then it was too late. The flood turned an AI the Forerunners used to control most of their military against them and had already absorbed tons of biomass. It was smart and powerful and getting stronger all the time. So out of desperation, after losing for a long time, they made the halos.

The flood by the time of the games is much newer and smaller. The gravemind is just as smart since it regains it's knowledge when it reforms, but it doesn't have the resources it did when it fought the Forerunners yet. It's only been active for like a few weeks really. It was still pretty contained to a few ships and then high charity, so by firing the Halo while it was still on the ark they were basically able to wipe out the majority of what was left of the flood. There's still some left, but they're sort of mindless without a gravemind to direct them.