A number of factors need to be noted with these words.
Fire appears time and again in Scripture as a reference to the presence of God (Genesis 15:17; Exodus 3:2; Exodus 13:21; Exodus 19:18; Acts 2:3). As Satan gathers the nations for a decisive assault on the church of Jesus Christ, God himself appears in fire as Commander (think Exodus 13:21).
Fire speaks also to the wrath and righteous judgment of God (Numbers 11:3; 2 Kings 1:10–12; Psalm 97:3; Isaiah 66:15–16). By fire the Lord destroys his opponent.
The arrival of divine fire means the battle itself is not engaged. Before the enemy can shoot an arrow, let alone rob the beleaguered city of food or water, the Lord destroyed the attacker.
9 And they marched up over the broad plain of the earth and surrounded the camp of the saints and the beloved city, but fire came down from heaven and consumed them,