I believe you would have to say that this is the first time John Wayne was not on the side of law and order in a movie since Three Godfathers. Between then and The War Wagon, a past that is less than savory has been hinted at, but only in The War Wagon has it been explicitly said he's an outlaw.
An outlaw with revenge on his mind. He's going rob Bruce Cabot, the slimy villain who's taken over his ranch and discovered enough gold on it to make him a rich man.
This is a caper film, maybe the only one Duke ever made. Though it might not come to mind, this film is definitely in the tradition of Topkapi and How to Steal a Million. Granted the comedy isn't exactly highbrow like the other two films, still the War Wagon is an honorable addition to that genre.
Helping Wayne along in his enterprise are Kirk Douglas a gunfighter/ safe-cracker, Howard Keel a cynical Indian, Robert Walker, Jr. a young alcoholic explosives expert and Keenan Wynn an old codger who works for Bruce Cabot and is essentially their inside man.
Kirk Douglas in his memoirs The Ragman's Son held the Duke in enormous respect even though their political views differed radically. The three films they did together show the good camaraderie they developed.
The title of the film refers to an armored vehicle with a Gatling gun that Bruce Cabot uses to ship gold. I won't say what the plan is on how the War Wagon is dealt with, but anyone who has watched the George Marshall/Glenn Ford film, Imitation General, will have some idea.
A good entertaining John Wayne western which is as good as it gets.