Because the Star Empire has a lot of internal strife right now,
We don't know that. The military seems pretty well in charge from my point of few.
and if the Federation and Klingons attacked them,
Why would they do that? Come now, the Federation never would, and I have my doubts about the Klingons. They are in a far worse shape than the Romulans.
they would be in a position that would be difficult to defend, to say the least.
I don't know about that. Remember, the SFCIII isn't canon. The Federations and Klingons too considerably more losses in the Dominion War than the Romulans. I personally believe no one is in a postion to fight anyone right now.
Romulans look for advantage, and will only strike when they think they have one. Romulans are chess players, as simple as that.
With the Federation, Klingons, heck probably the Breen and Cardassians breathing down their neck, they have a big incentive to sign
. Out of all the Alpha/Beta quadrant empires after the Dominion Wars, the Romulans are the ones that are best off. Plus with all the Scimitar, Valdore, and the supposed upgrading of the D'deridex (sourse: offical website), I wouldn't mess with them right now. God forbid the Romulans have anymore thelaron weapons. And even if they didn't sign whatever to keep the pressure off, they simply ignore it and go about their whatever their fancy in secert
In addition, we don't know a lot about the strength of the Romulan military. They are hiding something, but is that 'something' strength or weakness? If it's weakness they'd sign. If it's strength, the Alpha and Beta Quadrants are in trouble! What sourse gives you that idea?
Actually, that isn't true. Remember in TOS when the Romulans scrapped their Birds of Prey in favor of Klingon D-7s? Even they had to admit their ship technology was behind that of the Klingons, going so far as to scrap their navy in favor of a foreign-bought one. This was a stop-gap measure to discourage a Federation invasion, once the Romulans had discovered their latest and best ship design was no match for a Federation cruiser (see Balance of Terror). Of course, this didn't prevent Kirk from entering Romulan space and stealing a cloaking device right from under the Praetor's nose (figuratively speaking).
Actually, the only reason the Romulans got the D-7s at all was because the producers dropped the Rom BoP model and damaged just before the shooting of the episode. The Romulans weren't to happy about that, no sir. Stupid producers. The fact is, in ENT time, Romulan ships seem to be superior to either Klingon or Starfleet ships. And then in TNG Romulans technology generally seems to be ahead of the Klingons and about equal to the Federations. In Nemesis, it seems Romulan technology is pulling ahead of everybody elses.
To be certain, the Romulans do value science. Their quantum singularity drive is the envy of many a Alpha Quadrant power. The 'light bending' type cloaking device was also a great advancement that had even Spock impressed (as opposed to the much-inferior Suliban particle-dispersion cloak). But the Romulans value subterfuge and spying at least as much.
Romulans can rely on neither alone but on both.
Why reinvent the wheel of fire-when-cloaked tech, if you can spend a tenth of the resources on stealing it from the Klingons? Especially when those saved resources can then be spent on further improvements like the 'perfect cloak', and keeping shields up while cloaked? Why would you want a M1A2SEP fire control system when you could have a T-72 fire control system for alot cheaper? Because one does the job a heck of alot better than the other. How old is the Klingon cloak system anyway? About 100 years is? I am sure it would be obsolete by now.
Romulans always did have the most advance cloaks in the quadrant, who's to say the Klingons didn't steal the plans from a Romulan project in the first place? There isn't any canon to suggest one way or another.
Remember when a small Alpha Quadrant power known as Earth kept pissing off the then-technologically-superior Klingons? Now it's the Federation and Romulans who are technologically superior (as they both value science, to varying degrees. As opposed to the Klingons who don't value science nearly as much).
The Romulans have an advance culture, and they recognize the vaule and importances of science every bit as much as the Federation does.