MarkB
Legend
Thing is that it seems they've played with what the Prime Directive actually is, over the years. In the TOS days it was about not messing with less advanced (non star-faring) cultures so as not to alter their independent growth and development. Screwing with the Eminiar VII-Vendikar war wasn't mentioned as a Prime Directive issue, because they were both developed cultures. In later years it morphed into not interfering with *any* cultures, developed or otherwise.
The Prime Directive was always about not interfering with pre-Warp cultures. The Federation also avoids interfering in the internal affairs of other powers, but that's not part of the Prime Directive.