Albert Barnes Commentary 1 Kings 20:23

"And the servants of the king of Syria said unto him, Their god is a god of the hills; therefore they were stronger than we: but let us fight against them in the plain, and surely we shall be stronger than they." — 1 Kings 20:23 (ASV)

Their gods are gods of the hills. The local power and influence of deities was a fixed principle of ancient polytheism. Each country was considered to have its own gods, and wars were regarded as being, to a great extent, struggles between the gods of the nations engaged in them. This is apparent throughout the Assyrian inscriptions. Compare also 2 Kings 18:33–35; 2 Kings 19:12.

The present passage gives an unusual modification of this view. The suggestion of the Syrian chiefs may have been a mere politic device, since they were anxious on military grounds to encounter their enemy on the plain, where their chariots alone would be of much service. In the plain, the Israelites had always fought at a disadvantage and had proved themselves weaker than on the hills (see Judges 1:19, 27, 34).