During the TNG era, hm? I guess it depends on how you evaluate the Romulans and Klingons in terms of military strength. The one wild card here that fans tend to overlook is that despite several episodes dealing with Klingons and Romulans sprinkled throughout TOS, TNG, DS9 and even ENT, we still don't know a whole lot about either society. Some fans assume the Klingons have a large empire, but the evidence of that is rather sketchy. Fans tend to lean on the military progress of "Yesterday's Enterprise" and the DS9 era as evidence of Klingon abilities. While these are persuasive arguments, they are by no means full-proof. The Klingons were said to be "dying" after the Chernobyl-like explosion on Praxis, and were ready to make peace with the Federation. The Romulans, on the other hand, evolved from total isolation with one-trick-pony Bird of Prey-type attack destroyers in "Balance of Terror" to Klingon cruisers to massive Warbirds by the time of TNG. Fans tend to forget that Stefan DeSeve revealed in TNG's "Face of the Enemy" that TNG-era Romulan Warbirds are powered by a captured quantum singularity. That's quite a technological accomplishment, seemingly catapulting the Romulans ahead of the Klingons, Federation and perhaps others in the Alpha Quadrant of the Galaxy. One thing both Klingon and Romulan societies appear to have in common is that they are extremely insular. Despite talk about Klingon aggression, the Klingons biggest enemy appears to be themselves; they are apparently quite prone to internal intrigue and even civil war. And while the Klingon warships appear to be formidable and at times numerous, perhaps we should question to what extent that really infers the empire's size, industrial might (presumably required to build a formidable starship fleet), and of course overall military capability. It has been suggested that many Klingon warships are very old, even antiquated. Could the Klingons have simply accumulated a substantial number of spacecraft over time, retaining old ships they don't have the resources to rebuild/refit/maintain? Let's keep in mind that in TOS "Day of the Dove", Mara said the Klingon planetary systems were poor and needed to push outward to survive. One possibility never questioned in these fandom forums is how the Klingons built their space fleet if their resources were scarce, and how the Romulans came to acquire Klingon ships if Klingon resources were scarce. Could it be that the Klingons "outsourced" most of their components (hulls, equipment) and only built some sensitive parts themselves (warp drives, weapons, cloaks)? If so, and if the Klingons and Romulans did form an alliance, could it have been strictly a business transaction where they jointly cooperate with a third party (some distant civilization on the opposite side of their empires from the Federation perhaps) to acquire resources and share designs for basic ships, and then each empire is responsible for installing its own sensitive technologies inside the hulls? TNG-era Romulans deploy incredibly huge, incredibly powerful and advanced Warbirds. We have seen them deployed alone and in groups. We also saw other types of Romulan spacecraft, mostly post-TNG. None of this conclusively proves anything, except that the Romulans are taken seriously by the Federation as a threat. TNG showed us confrontations with the Romulans that suggested there is a Cold War-style standoff between the Romulans and the Federation. This reinforced the image of the Romulans as a threat. (If they were just a single hostile world like the Eminians in "A Taste of Armageddon", they might be taken as a serious security problem, but not necessarily as a Cold War standoff.) While the post-TNG movies did lend to the appearance that there is some diversity to Romulan society (apparently not just all-Vulcanoids), one thing we haven't seen much of from either the Klingons or the Romulans in TNG was any substantial amount of species that would comprise their empires. (Even the ancient Romans indoctrinated ancient natives of southern England into their empire to functionally serve and project power on that empire's behalf.) Instead, both the Klingons and the Romulans appear to deliberately isolate their societies, with very limited interaction with outsiders. (There are important, though minor, exceptions; apparently there has long been strong covert ties between the Vulcans and the Romulans; the notion of Spock as a dealer of Kivas and Trillium in the Klingon disputed area also was interesting) So perhaps it can be said that both the Klingons and the Romulan empires are much smaller than the Federation, but like the Eminians/Vendikans, each empire started from a single world and grew through building and aggression. As we have seen from the changes of each empire over generations, it is possible for even a small and weak organization of planets to build itself up from within to become a formidable power, even while remaining in isolation. If this were the case, either empire could be a formidable Cold War-style threat to the other or to the Federation while not being "that big". (Look at World War II-era Japan; a small island nation used technology and extreme aggression and empire-building to project power over much of the western Pacific.) In a sense, both the Klingons and the Romulans resemble the ancient Roman empire, or possibly a combination of the World War II axis powers (Nazi Germany, Japan, Italy). The 800 pound gorilla in the room, though, was the Cold War notion of M.A.D. (mutually assured destruction), hence the use of neutral zones. If the TNG-era Klingons and Romulans engaged each other in battle, who would win? That may be the ultimate question in comparing military might between the two empires, but it is mooted by the fact that such a war never happened, probably because both sides were aware of M.A.D.