"The Lord, the Lord Almighty, has a day of tumult and trampling and terror in the Valley of Vision, a day of battering down walls and of crying out to the mountains. Elam takes up the quiver, with her charioteers and horses, Kir uncovers the shield. Your choicest valleys are full of chariots, and horsemen are posted at the city gates." NIV translation
Isaiah sees the Babylonian army coming against Jerusalem and the fact that it is a judgment from God.
"The Lord stripped away the defenses of Judah, and you looked in that day to the weapons in the Palace of the Forest. You saw that the walls of the City of David were broken through in many places; you stored up water in the Lower Pool. You counted the buildings in Jerusalem and tore down houses to strengthen the wall. You built a reservoir between the two walls for the water of the Old Pool, but you did not look to the one who made it, or have regard for the One who planned it long ago." NIV translation
Isaiah sees that, when the Lord brings the Babylonian army against Jerusalem, they will look to their own efforts for safety.
The Palace of the Forest refers to Solomon's palace which had four rows of cedar pillars creating the impression of a cedar forest.
The storing up of water and strengthening of the wall refers to efforts that Hezekiah took when Jerusalem was being threatened by the Assyrians.
Remnants of this strengthened wall still exist and are called the "Broad Wall".
At the time, houses had to be torn down inside the city walls to make room for this very large wall.
Isaiah laments that they look to their own efforts instead of remembering what God had done to protect them from the hand of the Assyrians.
This was their "failure of faith" as they had the example of what God had done in the past but they still did not trust in Him for the threat at hand.
Our faith grows stronger as we remember God's action in the past and apply it to our current problems. We can choose to trust in God or to fear the people and things of this world but we cannot do both.