Albert Barnes 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)

Ahaz was threatened on all sides: on the north by Rezin and Pekah, on the southeast by Edom (2 Chronicles 28:17), and on the southwest by the Philistines (2 Chronicles 28:18).

To these external dangers was added the still greater peril of disaffection at home. A large party in Judah was “weary” of the house of David (Isaiah 7:13), ready to join the confederacy (Isaiah 8:6, 8:12) and to accept “the son of Tabeal” as king. Seeing no hope of safety unless he could obtain a powerful protector, and with Egypt being particularly weak at this time, Ahaz turned to Assyria.