Only the Valiant

Only the Valiant, also known as Fort Invincible, is a 1951 western film produced by William Cagney younger brother of James Cagney, directed by Gordon Douglas and starring Gregory Peck and Barbara Payton. The screenplay was written by Edmund H. North and Harry Brown, based on the 1943 novel of the same name by Charles Marquis Warren.

Following the American Civil War, peace is maintained in the New Mexico Territory by Fort Invicible, a fortification set up outside a mountain pass that blocks marauding bands of Apache. The Apache are able to eventually take the fort by cutting off its water supply then assault the fort when its garrison is at its weakest.Captain Lance arrives with a patrol soon after the battle and captures Tucsos, the charismatic leader of the Apache. Lances scout advises the Captain to kill Tucsos but Lance will not shoot a prisoner. ........

Source: Wikipedia