What advice did Isaiah give King Ahaz during the Syro-Ephraimitic War?

During the Syro-Ephraimitic War, when the kingdoms of Syria and Israel joined forces to invade Judah, the prophet Isaiah advised King Ahaz to place his trust in Yahweh rather than seeking external military intervention.1

Isaiah believed that the threat from the northern kingdoms would be neutralized by the Assyrians and that Judah did not need to form a dangerous alliance. When Ahaz acted with timidity and refused to request a sign from God, Isaiah provided one anyway, prophesying the birth of a child named Immanuel—meaning "God is with us"—as a sign that God would remain faithful to his Covenant people.1

Despite this counsel, Ahaz rejected the prophet's advice and instead appealed for aid to Tiglath-pileser III, the king of Assyria. This decision resulted in Judah becoming an Assyrian vassal, which led to the payment of heavy tribute and the introduction of foreign religious practices into the Temple at Jerusalem.21