The strange thing about Robert Altman's career is that he plugged away as a director for a decade and a half before he got his break. He did industrial and educational shorts, and a ton of episodic television in the 60s - has anyone seen his 12 episodes of Whirlybirds? or his nine episodes of The Roaring 20s?
Nobody noticed his first real features, Countdown and The Cold Day In The Park. There's no indication in his work from 1954 to 1968 that he's a particularly distinguished or interesting talent.
Then, after 15 people turned it down, Fox offered him Ring Lardner Jr's screenplay for MASH. Lardner would complain bitterly about how Altman and the cast had essentially trashed his script. He would also win his second Academy Award for it.
Suddenly, with MASH, Altman's mature style leaps fully grown in the space's of a hospital base in the midst of the Korean war - the overlapping dialogue, the restlessly prowling camera, the Brechtian use of the soundtrack to comment on the action, the studied use of anachronism. His stock company is born here in the presence of John Schuck, Bud Cort, Rene Auberhonois and, of course, Elliot Gould. MASH also shows his weaknesses, notably the studied hipster contempt that mars his worst films.
MASH launches a unique career in American cinema, and unleashes an astonishing influence. If you doubt the influence, watch MASH and then throw on George Lucas's American Graffiti or follow Nashville with Hal Ashby's Shampoo, whose star, Warren Beatty, gave his greatest performance in Altman's McCabe & Mrs. Miller.