Hast thou not heard, etc.? An abrupt transition, such as is common in Isaiah. From speaking in the person of Sennacherib, the prophet without warning breaks off, and returns to speaking in the person of Jehovah, as his mouthpiece.
"Hast thou not heard," he says, long ago; or rather, "that from long ago! have done this?" Art thou so ignorant, so devoid of that light of nature, which should "lighten every man that cometh into the world" (John 1:9), as not to know God's method of governing the world?
How that "from long ago," in his eternal counsels, he designs the rise and fall of nations, and the mode in which their destruction is to be brought about? Art thou not aware that conquerors are mere instruments in God's hands—"the rods of his anger" (Isaiah 10:5)—to work his will, and then to have his will worked upon them in turn (see Isaiah 10:6-19)?
Sennacherib seems to be really reproached for not knowing what he ought to have known, and might have known, if he had listened to the voice of conscience and reason. Now have brought it to pass, etc.
All that Sennacherib had done, he had done as God's instrument, by his permission—nay, by his aid. He had been the axe in the hand of the hewer (Isaiah 10:15), the saw, the rod, the staff, of God's indignation (Isaiah 10:5), the executor of his vengeance.
The very purpose of his being was that he should "lay waste (certain) defenced cities into ruinous heaps."