You may need to read the first reading several times to figure it out. In today’s world we just don’t think this way.
Isaiah is prophesying that God is going to ‘use’ Assyria as His tool, a rod, or a staff. And with this tool He is going to chastise or punish Israel. Does that sound like a loving and merciful God?
Granted, we do not always know the ways of God, but to punish or destroy Israel? But let us pause a moment. Israel has turned away from God. Not just in little ways, but great and evil ways. King Ahaz has led the people to worship the Baals. They no longer worship God in the temple. Because they no longer believe in God then when the Assyrians attack God will not force the Israelites to follow Him. They have chosen their fate.
However, the prophet Isaiah is also warning Assyria. What? Why would he warn the pagans? The Assyrians will conquer Israel and inflict a great punishment. But the Assyrians will demand more than what God plans for Israel. As the Assyrians gloat another force will come, the Persians, and for the evil the Assyrians have done, the Persians will become the tool of the Lord. During the time of the Persians God will direct the king of the Persians to rebuild the temple for the Israelites.
History always indicates how God is in control. He asks us to be faithful to Him. He is always faithful to us.
May the Lord bless you in the name of the Father and of the Son and of the Holy Spirit. Amen