Charles Ellicott Commentary 2 Chronicles 25:13

"But the men of the army whom Amaziah sent back, that they should not go with him to battle, fell upon the cities of Judah, from Samaria even unto Beth-horon, and smote of them three thousand, and took much spoil." — 2 Chronicles 25:13 (ASV)

But the soldiers of the army. —Literally, Now the men of the troop, whom Amaziah had sent back from going with him to the war, they fell upon, and so on.

Fell upon. —The verb used in 1 Chronicles 14:9 and 1 Chronicles 14:13: Spread themselves. Here it means attacked intending to plunder (Job 1:17).

From Samaria as far as Beth-horon. —The name “Samaria” is probably corrupt. Otherwise, we must suppose that the mercenaries first returned home and then, by order of King Joash, set out again from Samaria and invaded the northern districts of the kingdom of Judah. For “Beth-horon,” see the Note on 1 Chronicles 7:24.

And smote ... of them. —Of their inhabitants.