Looking at the universe as it stood, there were probably only three races that could seriously be united.
The young T'au, with their own immaturity and ignorance, still held fantasies about the galaxy. They had yet to figure out what scale of enemy the Imperium was. They merely thought it was two or three times larger than the T'au Empire.
Then there were the nearly extinct Aeldari. As long as their prophecies did not spout nonsense about humanity needing to be exterminated to ensure their species' survival, they could become an ally. Even the Dark Eldar could barely be counted, since in the end, all they wanted was to drain other souls so that they would not be drained themselves.
The future Death Army had absorbed quite a few Dark Eldar from Commorragh. But if they truly wanted to ally with humanity, they had better change their lifestyle. Otherwise, when a bolter was pressed to their foreheads, they should not blame the Imperium for being too ruthless.
Finally, there were the Necrontyr—the Necrons of today. If they encountered a relatively open-minded overlord like Trazyn, or signed an agreement with the Silent King, who was drifting beyond the galaxy in solitary seclusion, then they were not entirely impossible to negotiate with.
Aside from a few overlords with extreme ideologies, most Necrontyr could actually be reasoned with. The price, however, remained steep.
The Aeldari were like this: there was a bomb in your house, and it could affect their house too. So they broke into your home, and when you discovered them, they gave you a whole speech about innocent bystanders and mutual destruction, yet never mentioned the bomb in your house that was about to explode.
In the end, the bomb exploded. Both you, who lived in the house, and the