In the spirit of all this war talk lately and also because I like the point Malchik brought up...

If the Dunmer began a campaign to conquer Tamriel, what unions would you see forming to fight against them?

In my opinion, Orcs and Nords would stand together against the grey skinned threat and most likely work with the Emperors forces, these 3 nations have reasons to hate the occupants of Morrowind. Nords because of their loss of morrowind to the elves, Orcs, well, they still have no province to call their own, this would buy them leverage from the Empire. I'd imagine the imperial senators and such must have a deep loathing for Morrowind, because they never officially conquered through force of arms, rather through negotiation.

The beast races might take this opportunity as an uprising against their elven hunters...or would wait until the war was over before attacking the weakened victor.

What of the other elves? Or the redguards? I'd like to hear your opinions.

I believe that the other races would fall behind whoever promised them more. Or just stay in their own lands, protecting their own interiors. The Orcs would probably follow whoever promises them more land. THe Imperials and Nords, I believe with no doubt, would take up arms against the Dunmer.

In my opinion this would be the sides.

I'm afraid the Dunmer would stand pretty much on it's own there. I cant see the High Elves and Wood Elves join up due their lack of numbers and fear for the final outcome. Besides not every wood elf or hig elf is very fond of the Dunmer.

The ones the Dunmer would defenitly face would be the Imperials sided by the Orks. These two races have become very friendly and will defenitly fight side by side. It is universaly known that Orks and Elves arent best friends. The Nords would join up with this alliance for the reason stampede mentioned. This is a golden opportunity to get revenge.

This alliance would even get more support, the argonians and Khajids will find this ideal to get rid of the slavery.

I dont see the Redguard interfere anywhere in this possible conflict.
The Bretons are neglible in this conflict.

a Dunmer nation is impossible. Even if they would try I dont see them win from the alliance.

I think that maybe the Bosmer and High elves woud stand together. And i think that they maybe coud stop the dunmer if the orcs and nords stand together and fightet the dunmers to.

The Bosmers and Altmers would not join the Dunmer
They might join the imperials. And the Bretons would probobly
Stay out of it or start a war with the redguards