Far Isle was a human colony planet, within Unified Earth Government space. The colony was the site of what is considered to be one of the United Nations Space Command’s worst atrocities; in response to a rebellion in 2492 that they were unable to quell, the UNSC razed the colony using nuclear weapons, leaving no survivors.
Spartans were originally made to fight rebels.
also in-lore the UNSC nuked an entire planet because they couldn’t stop a rebellion on it
https://www.halopedia.org/Far_Isle