John Gill Commentary Jeremiah 21:14

"And I will punish you according to the fruit of your doings, saith Jehovah; and I will kindle a fire in her forest, and it shall devour all that is round about her." — Jeremiah 21:14 (ASV)

But I will punish you according to the fruit of your doings ,
saith the Lord
The situation of their city, and the strength of its fortifications, however sufficient they might be thought to keep out an enemy from annoying them; yet it was impossible to hinder the Lord's coming among them, as he here threatens to do; and "visit" them, as the word signifies, in a way of wrath and justice, according to the demerit of their sins, expressed by "the fruit of their doings"; their punishment was the reward of their unrighteousness, the effect of their sinful practices; and, though this was dreadful and terrible, they could not but own it was just and equitable: and I will kindle a fire in the forest thereof ;
not in the forest of Lebanon, but in the city of Jerusalem; whose houses stood as thick as trees in a forest, and which many of them, at least the most stately, might be built or ceiled with cedars from Mount Lebanon and its forest; though some understand this of the cities and towns about Jerusalem; and so the Targum renders it, "in its cities"; and the Syriac version, "its towns"; but these seem rather meant in the following clause: and it shall devour all things round about it ;
the mountains and trees upon them, the cities and towns adjacent.