Marines guarded the US Mail trains in the 1920s after a rash of robberies.
Navy Secretary Denby, a former Marine, sent a message to the Corps, which reads in part: “
You must, when on guard duty, keep your weapons in hand and, if attacked, shoot and shoot to kill. There is no compromise in this battle with bandits. If two Marines guarding a mail car, for example, are suddenly covered by a robber, neither must hold up his hands, but both must begin shooting at once. One may be killed, but the other will get the robbers and save the mail. When our Marine Corps men go as guards over the mail, that mail must be delivered or there must be a dead Marine at the post of duty. -”
These guys did exactly what he ordered.
I guess the order was never rescinded.