Board Thread:Warhammer 40k General Discussion/@comment-65.29.225.205-20130427160705/@comment-15803608-20170610065251

The Gathering Storm is  shaking up the status que, and assuming the Eldar and the Imperium both survive I can see these historical events giving them the chance to set aside their differences and come together in a permanent Alliance.

From the Imperium Perspective, the Eldar saved Guillimon. Yvraine and her followers played an essencial part in bringing the Primarch back from the brink of death, and were it not for them he would have been finished off by the Black Legion. If the Imperium survives, then it will be because of Eldar Help. If Saint Celestine and Roboute Guilliman vouch for them, then the Imperium will accept this because then it's just The Emperor using his mouthpieces to give new revelation. The Imperium already has a mechanism for working with xenos - sanctioning. Just give a Sanction to the desired Craftworlds and Exodite Worldsand suddenly working with Eldar is no longer heresy. The Imperium can even keep it's xenophobia - there will always be Orks and Necrons and Tyranids to kill.

From an Eldar Perspective, they will need the help of the Imperium in order to survive. The Eldar are too outnumbered to face off all the ruinous powers alone, and even with the help of every Eldar to ever exist Ynnead might not have the strengh to kill Slaneesh - especially not if the her three brothers fight alongside her (after all, a thread to one of them is a threat to all of them). But if Ynnead and The Emperor (i.e. his Warp Presence) fight side-by-side then they might have a chance. If that's the case then the Eldar will only have staved off extinction with the help of Humans. It also helps that the Pre-Fall Eldar didn't use that many planets and since current Eldar use wraithbone for their resources: these two facts mean that there will be no need to compete with the Imperium for resources or real estate.

I don't see a Union Of Empires or Open Borders or Trade or Mixing Of Populations, but I do think a permanent Alliance (in which they agree not to attack each other and to come to each other's aid in times of need) could potentally come into effect. If not based on gratitude and sharing a fight for survival, then at the very least on mutual benefit.