Why does the lament sound like it describes something that had already occurred?
Isaiah 15:1 (ESV)
1An oracle concerning Moab. Because Ar of Moab is laid waste in a night, Moab is undone; because Kir of Moab is laid waste in a night, Moab is undone.
Isaiah uses the prophetic perfect tense—it is typical of prophecies to place us in the future as if the events described already happened. This indicates the certainty of that which is to come.
1 An oracle concerning Moab. Because Ar of Moab is laid waste in a night, Moab is undone; because Kir of Moab is laid waste in a night, Moab is undone.