Any Star Trek: Deep Space Nine aficionado familiar with the Dominion War can tell you that the conflict took place between the Federation Alliance (the United Federation of Planets, Klingon Empire and Romulan Star Empire) and the Dominion, Cardassian Union, Breen Confederacy, and Son'a Command. However, would that answer be correct? Following Damar's defection, the Dominion installed Legate Broca as the new leader of the Cardassian Union. When the Female Changeling recommended falling back to Cardassia to regroup in the DS9 episode "The Dogs of War," Weyoun theorized the Federation would be timid and leave them alone. Broca replied, "But what about the Klingons, and the Romulans, and...," at which point he was cut off. Was Broca about to mention an as yet unnamed member of the Federation Alliance? If so, who could it be?
In the episode "Valiant," Jake Sisko believed that Starfleet was sending Nog to visit the Grand Nagus of the Ferengi Alliance to deliver a proposal which would allow the Ferengi to side with the Federation. While the mission was sidetracked in that particular episode, were further attempts to bring the Ferengi into the war made? And were they successful? After all, the Dominion had previously taken Grand Nagus Zek's lover Ishka hostage, and the outcome of the war would influence systems throughout the region. It is possible that these factors convinced the Ferengi that aligning with the Federation would be more beneficial (and profitable) than remaining neutral.
The Son'a were known to manufacture ketracel-white for the Dominion's Jem'Hadar soldiers at some of their colonies, including at Devos II. Did the events of Star Trek: Insurrection convince the Son'a to switch sides? While their reunification with the Ba'ku might provide excellent motivation to ally with the Federation, the evidence to support such a move is not as clear. The mention of ketracel-white production for the Dominion on Devos II in "Penumbra" occurs after the events of Insurrection. It is also possible that Devos II was one of only a few Son'a colonies to continue their support of the Dominion, or that the Dominion had subjugated the colony and forced them to produce the much-needed drug.
Bajor signed a non-aggression pact with the Dominion prior to the Second Battle of Deep Space Nine, but the planet did so only at the urging of Captain Sisko. The Federation retook the Bajoran station following Operation Return, and afterward DS9 served as a critical base for the Federation Alliance. Most importantly, it was revealed in "Image in the Sand" that the Bajorans had allowed a Romulan military hospital to be built on one of the many Bajoran moons. The Bajorans clearly sided with their Emissary and the Federation, but their small fleet of starships were very limited in their capabilities. It is unlikely that the Dominion ever mentioned them in the same breath as military equals with the Federation, Klingons and Romulans.
The Dominion retreat to the Cardassian system came soon after Damar revolted and created the Cardassian Liberation Front. The Cardassian rebels launched several successful attacks and seemed to be on the cusp of causing more problems for the Dominion. However, Damar was betrayed, and the Dominion announced they'd crushed the rebellion by destroying their bases. That announcement came several scenes before Broca's remarks about Federation allies in "The Dogs of War." Since the rebellion was considered by the Dominion to be at an end, and the Cardassian fleet had yet to switch to the Federation's side, Broca was probably not going to mention the Cardassian Liberation Front as a Federation ally. The worry that more Cardassian soldiers and citizens could be drawn to the rebel cause was a concern for the Dominion, but as with the small Bajoran military, the Dominion did not anticipate entire fleets of starships eventually siding with the Federation Alliance.
Odo mentioned that the Tholians and Miradorn, like the Romulans and Bajorans, signed nonaggression pacts with the Dominion in "Call to Arms." When it appeared as if the Dominion betrayed their pact with the Romulans, it is possible it jeopardized the stability of their pacts with the Tholians and Miradorn. A Changeling was responsible for the death of a Tholian observer during a bombing at the Antwerp Conference in "Homefront," but that did not prevent the signing of their pact over a year later. Neither the Tholians nor the Miradorn merited mentions after "Call to Arms," so their treaties with the Dominion may have remained in place.
Little is known about the mysterious Tzenkethi race, except that they had previous disputes with the Federation and lived in a region of space near DS9. The Dominion sent a Changeling to infiltrate the Defiant and eradicate a Tzenkethi colony in order to create another rift between the Federation and Tzenkethi in "The Adversary." The plot was unsuccessful, but it remains unknown if knowledge of the events became public or were provided to the Tzenkethi. Their proximity to DS9 and possible awareness of the Dominion plot may have inclined the Tzenkethi to forget their previous animosities with the Federation and join the fight against the Dominion.
The potential exists that the mystery ally was a never-before-mentioned species, or even a lesser power like the Orions, Gorn or Nausicaans. The Orion Syndicate collaborated with the Dominion in "Honor Among Thieves," but the stance of the official Orion government was unknown. Following Captain Kirk's fight with the Gorn captain, little indication was given about the status of Gorn-Federation relations, other than the fact that a human colony existed on the formerly contested planet in the 2370s. Nausicaans were well-known mercenaries, but the series never explored their government. However, it would make sense for Orion, Gorn, or Nausicaan starships to be absent from scenes of the Federation Alliance fleets, since their own militaries may have been too small to contribute to the war's major battles.
Considering the information above, the two most likely candidates are the Ferengi and Tholians. Enough incentive existed for either government to enter the war, despite the fact that neither was directly mentioned or depicted as being part of the Federation Alliance. In addition to the Dominion's capture of the Ferengi leader's lover, the Federation actively courted the Ferengi as allies. The Tholians had a nonaggression pact with the Dominion, but tensions over the murdered Tholian diplomat and the dissolution of the Dominion's pacts with Romulus and Bajor could have eroded this relationship. Quark, Rom, or Nog would've almost certainly mentioned any Federation-Ferengi pact in dialogue at some point if one had been established, so if forced to choose, my guess is that the mystery ally was inevitably the Tholian race.
Who do you believe it was?
Jay Stobie is a science fiction writer who admits he has a perfectly normal obsession with Star Trek. He can be found on Twitter at @CaptStobie.