What does 1 Kings 20:28 mean?

A man of God came to the king of Israel and said, 'This is what the Lord says: The Arameans think I’m only the God of the hills and not the plains. So I’ll hand this huge army over to you, and you’ll know that I am the Lord.' 1 Kings 20:28 - Modern Text Bible

(God sends another message: He’ll give Israel victory again, to prove He’s not limited by geography.)