Albert Barnes Commentary Isaiah 37:26

"Hast thou not heard how I have done it long ago, and formed it of ancient times? now have I brought it to pass, that it should be thine to lay waste fortified cities into ruinous heaps." — Isaiah 37:26 (ASV)

Hast thou not heard - This is evidently the language of God addressed to Sennacherib. It is designed to state to him that he was under His control; that this was the reason (Isaiah 37:27) why the inhabitants of the nations had been unable to resist him; that he was entirely in His hands (Isaiah 37:28); and that He would control him as He pleased (Isaiah 37:29).

Long ago how I have done it - You boast that all this is by your own counsel and power. Yet I have done it; that is, I have purposed, planned, arranged it long ago .

That thou shouldest be to lay waste - I have raised you up for this purpose, and you have been entirely under my control (see the note at Isaiah 10:5).