Isaiah said to them, “Say to your master, ‘Thus says the LORD: Do not be afraid because of the words that you have heard, with which the servants of the king of Assyria have reviled me.
The contents of Rabshakeh's speech being brought to Hezekiah, one would have expected (and it is likely Rabshakeh did expect) that he would call a council of war and it would be debated whether it was best to capitulate or no. Before the siege, he had taken counsel with his princes and his mighty men, Ch2 32:3.
Commenting on 2 Kings 19:1-7
COMFORTED BY ISAIAH. (Kg2 19:6-7) Isaiah said . . . Be not afraid--The prophet's answer was most cheering, as it held out the prospect of a speedy deliverance from the invader. The blast, the rumor, the fall by the sword, contained a brief prediction that was soon fulfilled in all the three particulars--namely, the alarm that hastened his retreat, the destruction that overtook his army...
2Ki 19:5-7 Isaiah replied with this comforting promise: Hezekiah was not to be afraid of the blasphemous words of the Assyrian king; the Lord would frighten him with a report, so that he would return to his own land, and there would He cause him to fall by the sword.
Commenting on 2 Kings 19:5-7