The idea introduced by Star Trek that one civilization has no business of interfering in the affairs of another, even if what the other civilization is doing is perceived as outright wrong or evil.
The idea presently remains the domain of science fiction, as AFAIK no country has made it its foreign policy at this time.
Historically, the idea did exist and was practiced by India in the time of King Asoka after his conversion to Buddhism. He decided not to interfere in the affairs of any country, but to live in peace and harmony with all. Did it work? Yes: None of the neighboring countries viewed it as a weakness or as an excuse to attack India which would have been an easy prey at the time.
The idea can also be seen in Thomas More's Utopia.
If I were the President or King of a country, I would make the Prime Directive into a fundamental rule of my foreign policy. Those who disagree, don't worry, I have no intention of becoming a President (or King).
Why?
It may seem a bad idea at times. Suppose some evil dictator rules some country. Is it right for me to sit idly by? Good question. But I am a strong believer of not imposing my beliefs on others. Who am I to decide who is good and who is evil? For example, I am a vegetarian. Does that mean (if I were President) that I should ban meat? Certainly not!
When it comes to affairs of other countries, the situation gets even more complex. People in different countries have different sets of values. Should I impose democracy on Iraq, for example? Isn't that up to the people of Iraq to decide?
Now, you may say, how are the people of Iraq supposed to decide it when they do not have free elections. Well, I believe such an argument understimates the power of human will. I grew up in a country that had a strong tradition of democracy (Czechoslovakia) but which was not a democracy at the time I grew up in it (which, by the way, was the case because some other country decided what was right for us!). We all hated it! We despised Communism. And we did all we could to get rid of it. And we did get rid of it. It may have taken us thirty years, but we did it. Yes, WE did it, and because of that the freedom we regained is ours. Not a "gift" from some superpower.
If the people of Iraq (just using Iraq as an example, I don't mean to pick on them!) want democracy, they will have it. If they don't want it, well, that is their choice. Maybe it's a wrong choice, but whoever said people only have the right to make correct choices?
That is why I am a great admirer of the Prime Directive.