Robert Maxwell wrote:
Non-interference is the rule unless and until someone asks for help. If the Ba'ku sent out a general distress call while being attacked or forcibly moved by the So'na, the Federation would have more than adequate justification to intervene.
The Federation Council would seem to disagree after refusing to get involved in the Klingon Civil War until outside influence was found.
Since they're the same race and are fighting over resources, I can't see the Prime Directive being interpreted as anything other than "the Ba'ku are on their own".
Them being the same race was not generally known outside the So'na and the Ba'ku. I doubt the Ba'ku would mention that little detail when calling for help. By the time the Federation figured it out, the Ba'ku would probably be safe.
It could also be argued that, while the same species, the So'na became a politically distinct entity, which makes their return akin to a hostile invasion rather than a civil war/blood feud.
The Federation didn't get involved in the Klingon Civil War because it was primarily an issue of political succession--clearly not something the Feds had any business meddling with. If the So'na showed up to outright slaughter the Ba'ku, that's a totally different kettle of fish, and the Feds would be right to help when asked. If the PD says the Feds can't
intervene in such a situation, it's incredibly inhumane.