So What Happened To Phase II?
Well much as Star Wars was the cause of Phase II being developed in the first place, it was another science fiction film which ended it. You see, Close Encounters of the Third Kind was released and proved to Paramount executives that science fiction films other than Star Wars could be successful at the box office. At the same time, Phase II was already shooting test shots, costumes had been designed (essentially the same as seen in The Original Series) and sets were built. But it was expensive, and Paramount were feeling the pinch for what was only a television series after all. Personally I think they read the pitch for The War To End All Wars and got scared. So to re-coup their losses on the television series when they decided to cancel it, they sought to make the pilot into a film. Several tweaks later, new costumes made, the return of Leonard Nimoy... and you have Star Trek: The Motion Picture. It's more complicated than that, but you get the idea. A major change they made was to ditch the Phase II costumes and bring in a new one piece uniform in pastel shades... which wasn't for the better. But can probably be glad that we didn't get Phase II. If it wasn't for the films and the success of The Next Generation in syndication then we wouldn't have a franchise today. The Klingons could have become a hopeless mess, and did we really want to take The Original Series into the 1970s anyway? I think the key thing to look at here for illustration is Galactica 1980. That lasted all of ten episodes. I can't imagine that Phase II would have been much more successful. However, if you really want to watch episodes of Phase II then you can now, because there is the fan production Star Trek: Phase II (formerly known as New Voyages). They've done The Child and Kitumba so far. The only difference there is that they have both Spock and Xon, but no Decker or Ilia.