In requesting that Hephaestus, the god of blacksmiths, make her son Achilles a shield, the goddess Thetis expects him to produce a piece of work that will glorify and romanticize war. In actual fact, however, she gets a lot more than she bargained for. Because what Hephaestus produces shows war in all its grim reality, with all the immense suffering and desolation that it brings.
All of the evils depicted on Achilles's shield—rape, violence, and crucifixion—come about as a direct consequence of war. Yet in the ancient Greek culture in which the Iliad is set, war was still regarded as a noble endeavor, something to be venerated and eulogized as the supreme expression of what it meant to be a man.
Auden, however, by way of an imaginative reconstruction of the forging of Achilles's shield, forces a Greek goddess to confront the sordid realities of war in all their horror. Thetis is shocked by what she sees and is terrified to learn that her son Achilles—the "Iron-hearted, man-slaying Achilles"—despite his prowess in the arts of war, will soon be killed.
Upon learning of the terrible truth, Thetis cries out in despair. Perhaps she's finally come to realize, at long last, that war is not glorious or romantic, but the ultimate expression of evil.