The Dunmer may not be a threat in the Imperial sense, but I suspect that the Hist provoked invasion of Morrowind was engineered by the Aldmeri. The Dunmer were tainted, fallen, choose your adverb in Thalmor eyes.....and they were better fighters. Had a mindset that wouldn't turn away from atrocity if it came down to it. And despite the racial arrogance, they seem to be the one mer species that gets along the best with the majority of the humans in Tamriel. Notice how coincidentally the Dominion attack on the Empire occurred -after- Morrowind was devastated. There was no particularly large Dunmer presence in the Legions that we know of, but the Argonian invasion certainly prevented Morrowind from reacting and sending a fighting force against the Dominion...and they are elves. They think like elves....which would damage the Dominion's favored tool. They have the potential lifespan of an Altmer.....so none of this 'General So-and-So died, and we don't know the trick he used to save his bacon!'.
There's more going on here than we know or suspect....
4E 0 — End of the Oblivion Crisis.
4E 1 — Black Marsh secedes from the Empire
4E 5-6 — Red Mountain erupts; Vvardenfell is destroyed the The Argonians invade soon after.
4E 10 — Potentate Ocato is assassinated by Thalmor agents,
4E 29 — The Aldmeri Dominion is recreated
4E 171 — The Great War begins
The Argonian invasion occured 165 years before the Great War.....