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Could they take on the Klingons? Almost certainly. The Klingons were at war w/ the Feds, then the Cardies, then the Dominion. They've not had a good few years recently. Besides that, they probably weren't as strong as the Feds to begin with. I don't think the Klingons are in a position to take on any of the major powers single-handedly right now (bearing in mind that the Cardies don't really qualify as a major power anymore).
Now, to address what you actually asked, could they take on the Romulans and the Klingons? Both at once? I really doubt it.
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Col. Maybourne: "Teal'c... It's good to see you well."
Teal'c: "In my culture, I would be well within my rights to dismember you."
-Stargate SG-1: "Touchstone"
Of course, the likelihood that either of the two would pick this moment to invade is small. The Klingons have worked off enough aggression to pacify them for a few decades, and the Romulans will (especially in light of their casualties) prefer to resume their game of political chess until they recover.
And recall the credo of the Shadows....
The Federation of circa 2380 will be stronger than ever, and given its industrial base and technological superiority will continue tobe the "Alpha male" of the Alpha/Beta Quadrants.
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Gene: "I AM Star Trek"
Yvonne: "You can't sum yourself up in so small a package."
Gene: "SMALL?!!"
- Gene Roddenberry: The Last Conversation
In TOS and TNG, the Romulans often seemed like underdogs, at first very primitive and then greatly outnumbered, so that their only hope for victory was guile and the use of others to do their dirty work. I doubt the Romulans increased their absolute military strength in direct response to the war (that would be too altruistic of them), but their relative strength probably is now almost but not quite on par with the UFP and the pre-war Klingons. The Klingons probably have the resources and the will to rebound rather quickly, though.
But what about the other players? So far, UFP has indeed behaved as if it is the "Alpha male of Alpha", but we know next to nothing about Tholians or Breen. Neither has been in an open 1-to-1 war with the Feds, so we don't know how strong they are. And Alpha is big: even if the UFP is 8,000ly across as they claim, that's peanuts compared to the roughly 50,000 by 50,000 by 5,000 ly volume of the whole quadrant. The true "Alpha male" may yet to be undiscovered...
Timo Saloniemi
I recall reading some Fan stuff proposing the Tholians as intergalactic refugees, who immigrated their entire civilisation to the Milky Way to escape a powerful enemy. Thus they are content to stake out a patch of turf, keep a low profile, and skin alive anyone who steps onto their soil.
The Breen are a bizarre choice for conquerors, as their main adveraries all colonise on Class-M worlds. Despite some nonsensical garbage to the contrary, the weight of evidence is the Breen are from a world of ice. They're probably furry! Anyway, they cannot survive without those suits on any main Federation world. Perhaps they're simply in desperate need of new resources.
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"You don't need eyes to see; you need VISION"
- Faithless / Reverence
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Death before Dishonor!
However Dishonor has
quite a disputed defintion.
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"Forgive me if I don't share your euphoria!" (Weyoun to Dukat, Tears of the Prophets)
Dax's Ships of STAR TREK
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"...it might be easier to study
ancient societies from distant orbit than it might be to sit next to the
Guardian of Forever with a tricorder." - Baloo, January 2000
As for the Tholians, the Feds already beat them in at least one war. Since we only rarely hear mention of this war, we can figure it probably wasn't a particularly difficult victory. I would say that, on the whole, the Tholians aren't really a major power. Perhaps they would fall into a class of "minor major powers".
I don't think the Breen alone are very strong at all. I mean, they had a weapon that would completely disable a ship w/ one shot. But they never used it until they joined up w/ the Dominion. I thik this is because they knew that, even w/ this "superweapon", they didn't have enough ships/manpower/resources/whatever to actually win a war w/ any of the major powers. And, now that they've lost so much in the Dominion War, I would say they are probably going to slink back into, for the most part, seclusion from the major powers. They'll probably beat up on some weaker races around them to keep their egos up, but I don't think the Feds will be getting much trouble out of them (nor do I think the Breen would be much trouble, anyway, even if they did attack).
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Col. Maybourne: "Teal'c... It's good to see you well."
Teal'c: "In my culture, I would be well within my rights to dismember you."
-Stargate SG-1: "Touchstone"
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"We set sail on this new sea because their is new knowledge to be gained and new rights to be won" John F Kennedy
members.aol.com/mfwan/index.htm
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"I promise you, Wilma, that not one man on this force will rest until the criminal scum that did this are behind bars. Now let's go get a bite to eat." - Frank Drebbin, Detective Lieutenant in Police Squad
Of course, you can say that the Feds would beat the Rommies because, since it's a TV show and the Feds are the main characters, they can't lose. But, if the writers were to create an full-scale invasion of the UFP by the Romulans, and they wanted to be at all realistic about it, the Rommies would have to win.
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Col. Maybourne: "Teal'c... It's good to see you well."
Teal'c: "In my culture, I would be well within my rights to dismember you."
-Stargate SG-1: "Touchstone"
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"Forgive me if I don't share your euphoria!" (Weyoun to Dukat, Tears of the Prophets)
Dax's Ships of STAR TREK
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"We set sail on this new sea because their is new knowledge to be gained and new rights to be won" John F Kennedy
members.aol.com/mfwan/index.htm
They can be very calm and peaceful one moment then savage and deadly the next. A race of extremes.
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"...it might be easier to study
ancient societies from distant orbit than it might be to sit next to the
Guardian of Forever with a tricorder." - Baloo, January 2000