So Ahaz sent messengers to Tiglath-pileser king of Assyria, saying, “I am your servant and your son. Come up and rescue me from the hand of the king of Syria and from the hand of the king of Israel, who are attacking me.”
2 Kings 16:7

In Isaiah 7, God promised Ahaz that he would defend Judah. There was no need to ally with Assyria. Ahaz refused to listen, though, and treated God and his Temple with contempt.