why is the poem anthem for the doomed youth called "anthem"?

An anthem is "a rousing or uplifting song identified with a particular group, body, or cause." Owen means to be ironic in that, in war, the end result is horror. The "anthem" in this case is a doomed youth. Boys go to war, mere virgins with rifles. Their innocence is doomed the moment they enter war and their "anthem" represents this doom.