> The Arbiter and his Elites perhaps?
The Arbiter wants unity of Sangheilios so they can rebuild and relearn, starting a civil war probably wouldn’t be within his interest, it would set them back years in recovering from The Great Schism. If not the Arbiter, I’m not sure any Sangheili would launch a war upon his own planet, that goes against the fundamentals of any Sangheili no matter his political views.
Just running down the list:
Unggoy - Unlikely, they haven’t been known for war mongering.
Kig-Yar - While they’re advancing in small increments from picking up technology from the remnants of the war, it’s unlikely they’d be brave enough to launch an attack on Sangheilios alone.
Jiralhanae - At this point, I feel like the Brutes are pretty divided. You have some working for the Sangheili, there’s also some drifting in space doing their own thing. I don’t feel like they really pose much of a threat anymore, much less launching an attack on Sangheilios. It’s possible, but I don’t feel it’s that likely.
Humans - Also unlikely. Keyes promised peace as far as he could control, his control stops where ONI begins essentially, and as far as we know, ONI isn’t interested in a confrontational battle. They already have operations underway. It could be the Insurrectionists, but seeing as how they wouldn’t gain much from launching an attack on Sangheilios (unless ‘hired’ by some external party), I don’t think they’d want to put themselves on the radar.
Forerunners - Big nope. While I doubt they have enough numbers (if any) to launch an attack, I also really doubt their first appearance and greeting back to life in the galaxy would be to launch an attack on a planet for no apparent reason. Their history leads up to a tragic “karma sucks” scenario. They suppressed life, then ultimately got suppressed in the end. Re-living their mistakes would be foolish, given their fate.
Precursors - Given the fact that they are a potential candidate for the new threat, I’m going to stick with nope on this one as well, seeing as how I don’t think 343 wants to give that away.
In the end, I want to say it’s the San 'Shyuum, but we don’t have much of an idea of where their current technological, or militaristic standing is at the moment. They have the highest motive because they were the masters pulling everyone’s strings. They probably still want to seek after The Great Journey and might see the Sangheili as a threat now, or grouped them in as heretics with the humans.