God told a prophet, Isaiah, to give a king military advice - ! Why? God wanted to protect the future Messiah's tribe. Israel and Judah certainly needed this protection because of pugnaciousness (civil war after 3 kings, invasion by surrounding countries). Ahaz didn't trust God's promise to protect, so he trusted in Assyria instead. God still gave him a sign although he refused one.Understand that your plans are in the LORD's hands. You have a brain, but He made that brain! All depends on Him, not on you. Looking for a sign at every turn isn't the way to go; listening, however, is. Trust, obey, love Him. Heed His voice; do not harden your heart to it.
Soli Deo gloria.