The second Tallarn war
might have included loyalist marines. I can't remember it being explicitly stated one way or the other.
Short story
Imperial citizens unearth strange artifact. Artifact is actually a dormant gateway to the realm of Slaanesh. Artifact slowly starts to activate.
Eldar farseers detect the activation. They don't know whether the imperial forces have been corrupted, and, really, don't seem to care. Eldar troops are ordered to wipe out the human defenders, reach the artifact, and stop it from opening a frickin' portal to hell.
Imperial forces put up waaaay more of a resistance than the Eldar expected. Eldar advance stalled.
Portal has time to open. Legions of Slaanesh emerge and start nomming souls.
Eldar
finally explain themselves to the Imperial forces. The two sides team up, blow up demons, seal portal, and live happily ever after*.
*Story ends. This is quite a convenient, because if the story continued it would probably have involved the Inquisition arriving to wipe everyone out.
The "
Theft of Lethidia" has Tyranids vs demons, and Eldar vs tyranids vs dark eldar. Other battles that fit that bill happened in the
Karkarodon System, and both on
Pandora Prime and
Medusa V.