Because the Syrians have said - God resents their blasphemy, and is determined to punish it. They shall now be discomfited in such a way as to show that God's power is every where, and that the multitude of a host is nothing against him.
A man of God - Evidently not the prophet who had spoken to Ahab the year before 1 Kings 20:13, 1 Kings 20:22. He probably dwelt in the neighborhood of Samaria. Now that Ahab and his army had marched out into the Trans-Jordanic territory, another prophet, a native probably of that region, announced God‘s will to them.