Albert Barnes Commentary 2 Chronicles 14:10

"Then Asa went out to meet him, and they set the battle in array in the valley of Zephathah at Mareshah." — 2 Chronicles 14:10 (ASV)

The "valley of Zephathah"—not mentioned elsewhere—is likely the broad wadi that opens out from Mareshah in a northwesterly direction, leading into the great Philistine plain. Upon Asa's advance, Zerah withdrew into the wider space of the wadi, where he could use his horsemen and chariots.