“But when they came to the camp of Israel, the Israelites rose up and attacked the Moabites, who fled before them. As they entered Moab, they continued to attack. They overturned the cities. On every good piece of land everyone threw a stone, until it was covered. They stopped up every spring of water. They felled every good tree. Only in Kir-hareseth did the stone walls remain, until the slingers surrounded it and attacked it. When the king of Moab saw that the battle was going against him, he took with him seven hundred swordsmen to break through, opposite the king of Edom. But they could not. Then he took his firstborn son who was to succeed him. He offered him for a burnt offering on the wall. Great wrath came upon Israel. They withdrew from him. They returned to their own land.”
When the Moabites arrived at the camp looking for spoils, the Israelites rose up against them so that the Moabites fled. They were expecting no resistance, when the Israelites attacked Moab. They fulfilled the prophecies of Elisha who said that they would take over their cities, throw stones on their fields, stop up the wells, and chop down the trees. Everything seemed to go according to plan. Only at Kir-hareseth, which was the capital of Moab, did they meet any resistance. Then the king of Moab sacrificed his first born son on the wall of this city. The Israelites were so impressed with this courageous act that they refused to battle more. They returned to their own land after this. The idea of human sacrifice was fairly common among ancient people. Just remember Abraham and Isaac.