One of the best American directors of the 60s and 70s.
He should have done more films then, but even in his best and most successful days he had a lot of trouble with the "Hollywood system", and not everything worked for him as smoothly as it should. He was fired by The Train and Altered States and in others like The Chase and The Missouri Breaks the ideas of the people involved differed too much from his. He made films against the system within the system, and had to pay for this.
But he did until the late 70s many things which were ahead of the time in genre films. Probably the 1st who used in a western (or in a Hollywood film) a slo mo shot in The Left Handed Gun, nearly 10 years before he shocked the audience with bloody slo mo violence in Bonnie & Clyde.
Watch as one example the last shot from Alice's Restaurant and you know of what he was capable, and how much he expanded the usual way of Hollywood filmmaking.
Bonnie & Clyde, Little Big Man and Night Moves are still excellent films.