"An oracle concerning Moab: Ar in Moab is ruined, destroyed in a night! Kir in Moab is ruined, destroyed in a night!" NIV translation
As we have seen in the last couple of chapters, God is dealing with Israel's neighbors and, here, He turns to the Moabites. Lot, the nephew of Abraham, had a son with his oldest daughter after the destruction of Sodom and Gomorrah (see Genesis 19). This son's name was Moab and the land of Moab is what we call Jordan today. Ar and Kir were the two strongholds of the land and we see, here, a prophecy that they will be destroyed in a single night. In our world today, we do not even wonder about the fact that a city can be destroyed in a night. But, at the time of Isaiah, they did not have the weapons of mass destruction that we have and so the only way that this would be possible is by the very power of God.