Charles Ellicott Commentary 2 Kings 3:23

"and they said, This is blood; the kings are surely destroyed, and they have smitten each man his fellow: now therefore, Moab, to the spoil." — 2 Kings 3:23 (ASV)

The kings are surely slain. — Have surely fought with (or destroyed) one another. Septuagint, ἐμαχέσαντο. The supposition was not improbable.

Confederates of different races often had been known to fall out among themselves (2 Chronicles 20:23, and Note), and in this case the old enmity of Edom towards Israel, and the suppressed jealousies between Israel and Judah, made such a result very likely.

The Moabites would also know that the wady had been waterless, so that their mistake was natural. Once their instinct for plunder was aroused, they did not stop to think, but with a wild cry of “Moab, to the spoil!” they rushed in disorder upon the Israelite camp.