As the fire burneth a wood - The same idea is here presented under another form. No image of desolation is more fearful than that of fire raging in a forest; or of fire on the mountains. As trees and shrubs and grass fall before such a flame, so the prayer is, that they who had combined against the people of God might be swept away by his just displeasure.

The flame setteth the mountains on fire - This may refer to the burning of the straw and chaff, after the grain was threshed and winnowed. And as their threshing-floors were situated often on the hills or mountains, to take the advantage of the wind, the setting the mountains on fire may refer to the burning of the chaff, etc., in those places. Let them be like stubble driven away by the wind, and burnt by the fire.

As the fire burneth the wood,.... Or "forest" (m); which is sometimes done purposely, and sometimes through carelessness, as Virgil (n) observes; and which is done very easily and swiftly, when fire is set to it; even all the trees of it, great and small, to which an army is sometimes compared, Isaiah 10:18, and as the flame setteth the mountains on fire; either the mountains themselves, as Etna, Vesuvius, and others; or rather the grass and trees that grow upon them, smitten by lightning from heaven, which may be meant by the flame: in like manner it is wished that the fire and flame of divine wrath would consume the confederate enemies of Israel, above mentioned; as wicked men are but as trees of the forest, and the grass of the mountains, or as thorns and briers, to the wrath of God, which is poured out as fire, and is signified by everlasting burnings.
(m) "sylvam", Montanus, Tigurine version, Junius & Tremellius, Piscator, &c. (n) Georgic. l. 2. v. 310.

Pursue them to an utter destruction.

The mountains - The woods upon the mountains, which in those hot countries, when they have once taken fire, burn with irresistible violence.