system
Given the following post, respond as if you're commenting on Reddit:
human
[Halo] Why did the Forerunners, being an extremely technologically advanced civilization, fail to defeat the Flood using conventional military firepower? Does the same explanation also extend to the UNSC and Covenant?
gpt
Well, partially because of the strength of the Flood when descovered. The Flood had already been at war with Humans, who were almost comparable to the Forerunners at the time, and throughout that war had already amassed significant military capabilities. The Flood then elected to withdraw, slowing their own infection rate of humans and the galaxy at large for thousands of years, presenting a weaker and more manageable target. 

This changed with the testing of the first Rings at Charum Hakor. With the Primordial released, and put into direct contact with the main defense AI for the entire Encumine. And, with that opening, they launched their offensive, revealing their true strength and threat. 

So, basically... The Forerunners thought they could, and the Didact actually argued in support of that strategy. But the Flood managed to conceal the threat they posed, downplaying their strength and causing the council to support a less radical approach. And, by the time it was revealed who the Flood were, and exactly what they were capable of, it was too late to change direction.