Ted you know that the watchmen story was written in the mid 80's meaning if the dark knight and the spirit seem similar, it more likey that they are imitations of watchmen but have the advantage of being made into movies first.
Which came first the chicken or the egg? Notice who worked on the Spirit Revival. http://willeisner.com/spirit/index.html
The egg! I think Watchman is supposed to be based on the comic books and serials of the previous decades except good and evil aren't so black and white, and the "heroes" act like humans. The move definitely has a '40's film-noir feel to it. You can find similar heroes with clay feet in many movies from the era. Not so much in children's entertainment. I got out of the service in 1980, and sweated out the Reagan years. I remember the paranoia. I don't think the movie's solution of a common enemy would have brought world peace.
There isn't one sir. I agree that a common enemy wouldn't solve all the world's problems but it would solve the most pressing problems given that current world's political stage.