The Halo response:
100 000 years ago humanity was a space faring race that posed a threat to Forerunner dominance. At a certain point, the Flood entered the Human controlled part of the galaxy. After a long and costly war, humanity not only beat back the flood, but cured it.
In their desperation for new worlds, uninfected worlds to settle, they started to expand into Foresunner territory. Also, Flood was silently infecting outer Foresunner worlds, and humanity was forced to cleanse them. Forerunners saw this as a act of agression and waged war on a weakened humanity.
After humanity was literally beaten back to the stone age, stripped of their technology and knowledge, the Librarian altered our genetics, thus splitting humanity into the many different races we know today.
Also, to ensure we wouldn't rise up as a unified humanity, she implanted a "geas" (genetic orders) to be distrustful of other humans, even violent towards each other. This is so that humanity would take longer and have a harder time to get back to their powerful, unified state 100 000 years ago. Basically, to prevent humanity from realizing its full potential once again, Forerunners made it so that humans will always rebel against each other.
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Sorry for the fictional response. But I see there are plenty serious ones all ready here.
Xbox: Best hardware, Game Pass best value, best BC, more 1st party genres and multiplayer titles.
