'Birth of a Nation' IS a cinematic milestone. It was the first epic. The camera work was revolutionary for its time.
I've never seen "Birth of a Nation", but I was loaned a VHS copy of "Triumph of the Will" by a college professor for a history class I took. I don't speak German, and it didn't have subtitles, so I missed what was said, but there's no missing the message Hitler, Goering, etal. were sending. An interesting sidenote is that John Ford, drafted into the Army after Pearl Harbor, used much of the movie for his own "Why We Fight" series.