system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] Has there ever been any case of chaos ever working with any of the other factions in the 40k universe?

I don't know why I ask, but I've seen Imperium/Tau/Eldar alliances, and it just crossed my mind, has chaos ever had an alliance with any of the other races, even if it was a sort shaky alliance that ended in betrayal?
gpt
Since the warp entities can't enter the material plane without being summoned and sustained by someone else then they have had 'alliances' with everyone. The chaos space marines are in a way allies of chaos through corruption or promises of power and almost every race has had some of its members corrupted at one point or another.