>>718957324
Well lets take a look at some of the most notable alien factions and see how well diplomacy would go?
>Eldar
Too fractured as a race that you'd have to have specific agreements with different craftworlds, with some outright refusing to come to the table at all, Biel-Tan being the most obvious. Never mind the Dark Eldar, who relish fucking up relations between Humanity and all Eldar with their raiding parties. Add onto that the Eldar sense of superiority and the casual nature in which they will attack because a farseer said something bad would happen centuries into the future and any agreement would be temporary at best.
>Tau Empire
These guys are the more insidious. They'll agree to NAPs without a problem, then attempt a cultural takeover of nearby Imperial worlds through offering technology, food, or other assistance to convince the planetary governor to change allegiances. Their manifest destiny ideology means they will without hesitation attempt to stretch the limit to what they're allowed to do diplomatically without being caught. Remember, they still think the Imperium is no where near as large as it is nor that the Emperor is a capital G God and not just a very important human that all the other humans apparently worship. Diplomacy could work but would be massively to the Taus advantage.
>Votann
The nuSquats came back and are thieving little shits who listen to whatever a super computer tells them to do. They might agree to negotiations but will automatically piss off the Admech when they start hoarding tech they want. Which is all the time.
>Necrons
Apparently this has already happened when the Bangles and a force from the Sautekh dynasty were fighting against the Tyranids(?) together. Possible, but like the Eldar you'd have to make specific agreements with specific dynasties and doing so might piss off other dynasties. Political quagmire no matter how you cut it as the Necrons are now constantly bickering amongst each other any way they can.
