John Gill Commentary 2 Kings 16:7

"So Ahaz sent messengers to Tiglath-pileser king of Assyria, saying, I am thy servant and thy son: come up, and save me out of the hand of the king of Syria, and out of the hand of the king of Israel, who rise up against me." — 2 Kings 16:7 (ASV)

So Ahaz sent messengers to Tiglathpileser king of Assyria , &c.] Of whom see (2 Kings 15:29) saying, [I am] your servant, and your 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 2 Kings 18:7) 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 7:4–16) .