Isaiah is talking about fear. The historical background is that Judah felt threatened by the presence of the Assyrian empire. They were sweeping down and conquering all of the nearby nations. Judah was frightened. As the people of God, what should they have done? In deep respect, awe, and faith, they should have cried out to God for His protection. He is the Lord of heaven and earth. He governs it all. He could easily have stopped the Assyrians. However, the people conspired with other nations, saying to them, in effect, "You protect us. We will ally with you. We are afraid of the Assyrians, but if we join together, we will have enough power to fend them off and be safe."
God looked at that as a conspiracy against Him. He was their Lord, their King, their God. He certainly had the power to protect. And thus, His instruction to Isaiah: "Do not enter into that conspiracy with them."
John W. Ritenbaugh
The Fear of God