So Ahaz sent messengers to Tiglathpileser king of Assyria,.... Of whom see 2 Kings 15:29
saying, [I am] thy servant, and thy son; signifying, that he would be his vassal, and become tributary to him, and serve him as a servant to his master, or a son his father, on condition he would come to his assistance, and so he became his servant; hence his son Hezekiah is said to rebel against the king of Assyria, 2 Kings 18:1
come up and save me out of the hand of the king of Syria, and out of the hand of the king off Israel, which rise up against me; which assistance he had no reason to call in, since the Lord had promised him deliverance from both those kings, and gave him a sign of it, Isaiah 7:4.