King Ahab of Israel had won a previous battle against the Syrians (1 Kings 20:21), apparently among the hills and mountains of Israel. The victory gave rise to the Syrian claim that the God of Israel was a god of the high country, not of the level plains. The Syrians reflected the typical pagan belief that different nations and different parts of the earth had their own regional gods.
The next battle against the Syrians would be fought successfully in the plains and valleys (1 Kings 20:20). This would prove that the one true God ruled over the entire earth.
1 Kings 20:28 – Because the Syrians have said, The LORD is God of the hills, but he is not God of the valleys, therefore will I [God] deliver all this great multitude [this vast army, NIV] into thine [Israel’s] hand, and ye shall know that I am the LORD.