The rise and fall of empires (cyclical barbarian invasions) set in a lecture hall

I read a short story a few years ago (late 90s) where a professor of Ancient History is giving his favorite lecture to his students about the rise of their empire. Some points I remember:

• The story is set in a packed lecture hall


• The lecture is about an ancient poem that commemorates the battle where the current empire captured Earth from the last empire.


• The lecture is interspersed with flashbacks from the invasion under discussion.


• In the flashbacks a professor of history discusses the rise and fall of the Roman Empire.


• The professor is excited to finish the lecture, but is interrupted right at the end and forced to read an announcement

My Google-fu has failed me. Does anyone have any clue as to the title or the author?