Based on the abilities of the powers shown during Deep Space Nine (specifically the Dominion War) I'm not sure any of the three major powers (I never considered the Cardassians to be on par with the Romulans, Klingons, and Federation) had the necessary numbers to completely
occupy one of the others, and certainly not
both of the others.
The Federation, well, in their case its a moot point; they wouldn't do something like that.
The Klingons couldn't handle taking over a demoralized Cardassia in DS9. Sure, they bloodied their nose a bit, they hardly
won during that conflict without opposition.
The Romulans forces were never made very clear... but one gets the impression they have small numbers of powerful ships rather than vast armies of smaller, less powerful ships.
So, really either way, I don't see a Klingon occupation of the Federation as something anybody was ever in any real danger of having happen; if somehow the Klingons marshaled the numbers necessary for it, the Federation would time travel to stop them (Maybe they'd send somebody from Kirk's Enterprise in a JJ Abrams film?)
-Withers-