There is the Prime Directive and the Temporal Prime Directive. I just wondered if there was ever a Dimensional Prime Directive? Picard once described that violating the Prime Directive, no matter how just it may seem, always caused harm. That can certainly be said of extra-dimensional interference;
1. Species 8472- The Borg entered into their dimension and nearly wiped out not only them, but pretty much the whole galaxy
2. Paralles- When the dimensional boundaries collapse and revealed a Enterprise that came from a Borg conquered universe, it tried to destroy Worf so he couldn't repair the rift. If it had suceeded, the multiverse could have been destoryed.
3. The Alternate Universe- where to begin....
A. The Defiant- by mistake, crossed over and drastically changed the course of history in the Alternate Universe
B. Capt Kirk- talked alternate universe Spock into bringing about a more passive Terran Empire. This resulted all of humanity in that universe to be slaves.
C. The Orbs- If not for Major Kira, the altenate Kira would have stolen a Bajorian Orb. Seeing how the Emissary and the prophets were so in tuned with the Orbs, a loss of one could have broken down everything.
Anyway....I never saw where there was a Dimensional Prime Directive...was there ever one in the novels??