Charles Ellicott Commentary 2 Kings 3:26

"And when the king of Moab saw that the battle was too sore for him, he took with him seven hundred men that drew sword, to break through unto the king of Edom; but they could not." — 2 Kings 3:26 (ASV)

The battle was too severe for him. —The garrison was giving way under the destructive fire of the slingers.

To break through even to the king of Edom. —Because the Edomite contingent seemed to be the most vulnerable point in the allied army, or because he hoped that these unwilling allies of Israel would allow him to escape through their ranks.