The Senario: While exploring the bajorian wormhole the federation encounters the empire.
Rules: Canon material only, no temporal interferance.
Timeline: Hight of Empire, just before the battlend of Yavin 4.
Deep Space 9 just before the first encounter with the Dominion.
Additionally: The Q set it up so they will no get involved, expect to annoy significant characters. The rebellion still exists but they are severely weakened as the empire will retask the death star and send a fleet of star destroyers instead. The klingons will quickly join the federation, the Romulus will but it will take time. The Cardassians will ally with the empire.
Honestly I didn't remember all of the details of all of the encounters with the Tholians. I actually thought that might be a stretch when I contrived it.
However, after your response I went and looked at this.
http://memory-alpha.wikia.com/wiki/Tholian
It seems that Sisko has a relationship with the Tholian Ambassador and is known to curry favors. So not out of the question.
For the Traveler we do know that he is able to boost his power with the empathy of others. So if you take a bunch of Betazoids along, who knows what they might accomplish.
I was referring to the Planet Killer that Matt Decker rendered inoperable in the original series. Since it was not destroyed, you have the possibility that some of it's systems are still accessible.
When I mentioned the Borg, I wasn't implying they would help. I was alluding to some bit of tech recovered from one of the encounters, most likely at Wolf 359.
I didn't even consider a ground battle, but though not the norm Starfleet and especially the Klingons are certainly capable of a ground assault at any scale.