The Prime Directive ONLY applies to civilizations that are not warp/FTL capable AND they must not have any knowledge of warp/FTL civilizations. If a civilization becomes aware of Warp/FTL civilization then the PD does not apply. PD would not apply to Romulans or Cardassians.
And that's why I keep scratching my head whenever people in-universe and IRL keep citing the PD for matters concerning the Romulans or Cardassians.
Except that's HOW the PD was interpreted during the ENTIRE run of TNG (No the PD wasn't like that in TOS. In TOS the PD only protected primitive civilizations who had to spaceflight capability and no knowledge that life existed on other worlds) - but during TNG's heyday, the PD was used to justify:
- Not letting Worf have PROOF the Duras family were traitors to the Klingon empire because Picard saw it as using Federation resources to effect political change in the Klingon Empire (Yes, he ultimately relented, but not before giving a long winded self serving lecture to Lt. Worf.)
- The Federation standing by and watching as Cardassia invaded Bajor and killed MILLIONS of Bajorans (and from various comments in various episodes on both TNG and DS9 -- the Bajorans were a space faring civilization before Earth was.)
To be fair, both of these examples make it seem as if the PD was just a handy blanket rule politicians and corrupt Starfleet brass could use to absolve themselves of taking responsibility for issues that would be politically or diplomatically inconvenient. It would be geopolitically inconvenient to cause a political crisis in the Klingon Empire, and Bajor, being a single-system civilization with no known strategic value is simply not worth the resources they'd have to sarifice for its liberation. If they were worth it, they would've fought the war until they captured Bajor, but they didn't. Ugly, dirty politics and self-righteous hypocrisy, just like it has always been.