“But the men of the army whom King Amaziah sent back, not letting them go with him to battle, fell on the cities of Judah, from Samaria to Beth-horon. They killed three thousand people in them. They took much booty.”
This may have been the start of the war between Israel and Judah. Certainly the disgruntled Ephraimites were looking for some place to fight. However, Samaria was the capital of Israel so Judah had no control of it. Beth-horon was in Benjamin or disputed territory with Ephraim. Actually this group did not stray far from their own area of Ephraim. They killed 3,000 people and took the spoils of war with them. This was some kind of strange punishment for King Amaziah for asking and then refusing these Ephraimites.