God shall rebuke them. Although the Assyrian armies under Sennacherib threatened to completely inundate Judah, the Lord intervened (see ch. 37:36). Instead of overwhelming Israel, the enemy would himself be overwhelmed.
The chaff of the mountains. The symbols used here aptly express the utter weakness and insignificance of the Assyrian armies before the power of God. One moment they were sweeping onward like the rushing waters of a mighty sea, threatening to overwhelm the land of Judah; the next, they would be like chaff or thistledown driven before the wind.
A rolling thing. Literally, “wheel,” but here supposed to refer to the wheel-shaped dried calyx of the thistle Gundelia toumefortii.