The poem describes the biblical destruction of the Assyrian army that attacked Jerusalem. In 3 sentences:
The Assyrian army arrived in Jerusalem gleaming in armor but was destroyed overnight by the Angel of Death who breathed a plague on the sleeping army, leaving the riders and horses dead on the field. All was silent in their camp the next morning with banners waving alone and weapons untouched, and the widows of Assyria wailed while their false gods lay broken, as the might of the Assyrians melted before God.