God's "I Wills"

Isaiah took lengths to describe Israel's sin and idolatry. How they rather trust in foreign allies like Assyria and Egypt, which were both destroyed and conquered when Babylon rose to power. And how God ended up judging the world for unbelief (Chap 24).

He also spent the 2nd half of the book (Chap 40-66) to describe how God will fulfill his promise to Israel, to establish them, despite of the great Sin that they lived for.

How then can we reconcile this to Isaiah 1:16-20? Did God just forgave Israel simply by exhausting his wrath from the foreign invasions (40:2)? Or did God required a specific set of actions from Israel in order to reinstate them back as the Messengers of his Laws (42:21)?

While a condition was set at the beginning of the Chapter, we also cannot deny the "I WILL"s in which God clearly said "I WILL" make it happen (42:6, 43:19, 44:22).

I believe the answer is here.

For I will pour water on him who is thirsty, And floods on the dry ground; I will pour My Spirit on your descendants, And My blessing on your offspring;
Isaiah 44:3 NKJV

Until the Spirit is poured upon us from on high, And the wilderness becomes a fruitful field, And the fruitful field is counted as a forest.
Isaiah 32:15 NKJV

The restoration of Israel, and thus all mankind to God is solely the work of God, the pouring out of the Holy Spirit. He will cause those who hear him, to repent, to believe in the Messiah, and live in obedience to him.