Um, remember a portion of the TOS PD was "No space flight capability AND (not or, and) no knowledge of life on other worlds".
Yeah, I remember - nothing of the sort was said in TOS. The only wording we ever got was "If they are at the 1960s levels of technology by some strange coincidence, don't carry your phasers outside your pants". And then they did.
So if a civilization had spaceflight OR already had knowledge of life on other worlds (regardless of technological capacity) the PD didn't apply.
Kirk never admitted to it applying no matter what, so we didn't quite find out whether that would have been true...
DS9 tells us the occupation began 60 years earlier and the Federation stood by and watched.
And also that the Bajorans stood by and watched, some of them apparently being very happy about being so nicely embraced by Cardassia.
Yes, they fought a open/hot war with Cardassia (wwhich was supposedly going on all throughout the first 3 seasons of TNG
...And possibly began at least a decade prior, too. I guess Bajor is a prime example of why the UFP shouldn't always interfere. It just ends in carnage that helps nobody.
Timo Saloniemi