Moabites were descendants of Moab, the son of Lot from an incestuous relationship with his first-born daughter after the destruction of Sodom and Gomorrah. Ammon, from whom the Ammonites descended was also an incestuous son of Lot’s younger daughter. There was a long-time history of bitter relationships between Israel/Judah and each of those nations.
Isaiah spoke the word of the Lord to Moab regarding their impending destruction. Moab’s destruction would be so great that the river would flow with their blood. The pride of a proud and haughty nation would turn into wailing. Their fields would be barren and there would be no harvest. Praying to Moab’s false gods would be a futile exercise. The Lord had spoken.