dulce et decorum est survived as long as the kind of classical infantry tactics (that Macedonian phalanx) that originally spawned it survived, and died when those tactics met their definitive end in the machine gun.

Yeah, that sounds horribly accurate -- you can't rip a great number of men to shreds from a distance without a machine gun. ....that, and the gas -- after all dulce et decorum est is called "the great lie" in a poem about gas attacks.