Can't find the following: In this story, published before 1979, Earth armies fight what they are told are aliens. They have a rallying cry as they attack the aliens (something like "Dirty Marts!" or somesuch) until one of the soldiers hears the attacking "aliens" screaming the same thing at him and his buddies. He realizes, too late if I recall, that they're fighting against other humans, and that the war was concocted by leaders to keep Earth armies fighting each other.

Any insights would be most appreciated. Thanks.