The Wild Bunch, but he's not quite there. One could argue Major Dundee (1965) is his test run for that later masterpiece. Both movies are Westerns about rugged men in a violent world holding on to their ideas of manhood, professionalism, and loyalty. Additionally, both feature many of the same actors in the supporting cast, including Warren Oates and Ben Johnson.
The main difference is the nature of the central relationship between the two male leads of each movie. In The Wild Bunch, Robert Ryan and William Holden were former partners and best friends forced to become hunter and hunted. In Major Dundee, Charlton Heston and Richard Harris are enemies who team up to track down renegade Apaches during the waning days of the Civil War.
Heston is Dundee, a glory-seeking Union officer in charge of a prison camp in New Mexico. Harris is Lieutenant Tyreen, the ranking Confederate officer imprisoned there and a former comrade of Dundee's. Tyreen is still bitter because Dundee was the deciding vote in a court martial that drummed him out of the service before the war.
Major Dundee was a troubled production, and Peckinpah fought with the studio over the budget and final cut. The version I watched is the "extended" version, which has some restored scenes and added different music from the version released in theaters in 1965 and is probably the closest we're going to get to Peckinpah's true vision for the film.
Much of the film takes place in Mexico, and down South, our heroes must contend with low supplies and occupying French forces. There's an extended siesta in a poor Mexican village and a romance between Dundee and an Austrian widow living there (Senta Berger). For a long time, it feels like we're in a completely different movie, and when a wounded Dundee recuperates in a city and descends into a drunken stupor, the movie stops dead.
Overall, Major Dundee has a rather episodic feel, unified by the unnecessary narration of Michael Anderson Jr. as a wet-behind-the-ears bugler. The narrative doesn't march with efficiency to the climax so much as it stops for incidents along the way: an night-time river ambush, the siesta with the Mexican village, robbing the French soldiers for their supplies, dealing with a deserter, etc. Many sequences are quite good, great even, but some characters feel shortchanged to the point I kept forgetting they were in the movie.
Again Peckinpah gives us a West that is more rough, more dangerous, and more grounded than what anyone else was doing in Hollywood at that time.While not as graphic as The Wild Bunch, Major Dundee doesn't pull its punches. Witnessing the aftermath of the massacre that kicks off the plot, we see the Apache did not spare the children and that one soldier was left hanging upside down and mutilated. Dundee says he hopes the man was dead when they did that to him, but one of his subordinates notes, "If he was dead, they wouldn't have done it."