In 1862, Union forces drive the Confederate Army of the Southwest into Texas, a development that renders the Santa Fe Trail a supply line to the Union's "bleeding troops." Hindering the delivery of the supplies, however, is a brash, cigar-smoking rebel captain named Vance Britton, who with his crack troop of Texas dragoons captures most of the Union wagons before they are able to reach Fort Point. Trader Sam McQuade, who supplies the nearby Apache Indians with dangerous guns and bad liquor, is disappointed when Col. Jeb Britton, Vance's brother, who has been assigned to put an end to the Confederate raids, arrives at the fort with only a small detachment of Union soldiers. McQuade believes that the government should enlist the aid of the Apaches in subduing the rebels, but Jeb, who is certain the Indians would indiscriminately kill Confederate soldiers as well as settlers, rejects the idea. That evening, Jeb meets McQuade's wife Julie, who recognizes him as the brother of her ex-fiancé, Vance. Believing that it was Jeb whom his wife loved, Sam tries to humiliate them both, prompting Julie to leave her husband and take up residence in the nearby town of San Gil. Jeb attempts to set a trap for the rebels, but his plan backfires, and he finds that his own brother is leading the plunderers. Vance steals Jeb's horses, boots and supplies, and when the embarrassed colonel returns to the fort, he learns that Sam has persuaded officials in Washington to negotiate with the Apaches. Soon afterward, Sam is attacked by Apaches and killed. Vance finds his body and discovers through a letter in the dead man's pocket that Maj. Thomas Riordan is on his way from Washington to meet with the Apache chiefs. Vance waylays Riordan, dons his uniform, and visits the Apaches himself, along with his men, sergeants Tucker and Calhoun. Vance is surprised to find that Chief Grey Cloud is actually Maj. Gen. Harrison Page, a white man who chose the Indian life in part because he married an Apache and because the government continually broke promises he himself had made to the Indians. Grey Cloud and three other chiefs decide to stay out of the white man's war, but just then word comes that a group of Apaches led by young Chief Geronimo has been arrested for McQuade's murder. Vance promises to get the Indians released, realizing that failure to do so will lead to battle. In San Gil, Vance, still posing as the Union major, encounters Julie, who angrily refuses to believe that he left her at the start of the Civil War to save her the pain of possible widowhood. He also visits the imprisoned Geronimo, who defends McQuade's murder by claiming that the trader's guns and liquor brought death to his people. Vance tries to discourage Delacourt, a bureaucrat newly arrived from Washington, from arming the Indians. Unsuccessful, he then decides to release the Apaches to appease Grey Cloud and plots to steal a shipment of Union gold that has just arrived in San Gil. Jeb appears, however, and although he is inclined to let Vance go free, he pulls a gun on him when he learns of Vance's robbery plan. Vance escapes anyway, and after he returns to his troops, they decide to head south, despite Vance's concern for the safety of the woman he still loves. Meanwhile, Grey Cloud accompanies a force of Apaches to San Gil, offering, under a flag of truce, to remain neutral if the Apache prisoners are released. An angry citizen insults and shoots Grey Cloud, and a fierce battle ensues. Vance and his men witness the attack on the town as they ride away, and the young captain orders his men to offer assistance. With the help of the Confederates, San Gil is saved. Julie returns to Baltimore, but promises to reunite with Vance after the war. Jeb then gives Vance a cigar, and after the two brothers shake hands, the Confederate soldiers ride away.