Im just realized that I dont really know why the sangheili in Halo 4 are enemys again. I think it might be connected to the book "thursday war"? Didnt finished it yet, but it sounds like both sides are just rearming
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They're basically a terrorist group. As far as I understand and remember from Glasslands, there's the group led by the Arbiter who don't worship the Forerunner, and then the group that still follows the Covenant religion.