The Arizona Kid (1939) on Netflix

Plot summary

In Missouri during the Civil War, Roy and Gabby scout for the Confederacy, and Val McBride commands a guerrilla band that flies the same flag while ignoring the rules of war. Riding into town, Roy runs into Dave Allen, a friend from his childhood who has joined McBride's riders, and Roy tells him McBride is no man to admire; Dave brushes the warning aside. McBride comes into the saloon where the two are talking, and he and Roy nearly come to blows before Union scouts ride in and McBride leaves with his men. Ordered by his superior to fight by the rules or lose his command, McBride rides off to wage war on both sides for himself. Roy and Gabby are then sent after him.