God's message to the king of Assyria is designed to shatter his arrogance and his illusion of absolute power. The king’s sweeping military victories and the widespread devastation he leaves in his wake are not the result of his own might. Instead, he is merely executing an ancient, preordained divine plan. The primary approach among commentators is that God established and prepared these exact historical events long ago, dating back even to the six days of creation [רש״י, ביאור שטיינזלץ, מצודת דוד].
When God asks the king if he has heard of this ancient plan, the question is posed with poetic irony. The Assyrian king never actually heard this truth, nor did he realize that he was acting merely as a tool in God's hands [שד״ל]. Yet, had he paid attention to the prophets of Israel who foretold these very events, he would have recognized God as the true architect of his conquests [מלבי״ם, רש״י]. God is the one who has now taken this ancient decree and brought it into reality, ensuring that the plan comes to pass exactly as intended [שד״ל].
As the king carries out this decree, fortified cities are reduced to desolate ruins. Most commentators agree that the resulting devastation leaves the once-mighty cities as abandoned mounds of rubble, eventually overgrown with weeds and flowers [רש״י, מצודת ציון, ביאור שטיינזלץ]. Others disagree with the imagery of blooming flora, maintaining that the descriptions refer exclusively to total ruin and destruction [שד״ל].
However, a distinctly profound perspective offers a completely different understanding of this devastation. The ruin of these cities is compared to a seed that must first rot in the earth before it can sprout a new plant. The cities were reduced to rubble so that new, fortified cities could ultimately blossom from the ashes. The true purpose of the destruction Assyria inflicted upon Judah was to awaken the people, inspiring them to return to God so they could rebuild their cities with even greater strength and glory during the reign of King Hezekiah [מלבי״ם].