A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
jer 21:1ZEDEKIAH CONSULTS JEREMIAH WHAT IS TO BE THE EVENT OF THE WAR: GOD'S ANSWER. (Jer 21:1-14)
Zedekiah--a prince having some reverence for sacred things, for which reason he sends an honorable embassy to Jeremiah; but not having moral courage to obey his better impulses.
Pashur--son of Melchiah, of the fifth order of priests, distinct from Pashur, son of Immer (Jer 20:1), of the sixteenth order (Ch1 24:9, Ch1 24:14).
Zephaniah--of the twenty-fourth order. They are designated, not by their father, but by their family (Ch1 24:18).
jer 21:2Nebuchadrezzar--the more usual way of spelling the name in Jeremiah than Nebuchadnezzar. From Persiac roots, meaning either "Nebo, the chief of the gods," or, "Nebo, the god of fire." He was son of Nabopolassar, who committed the command of the army against Egypt, at Carchemish, and against Judea, to the Brown prince.
according to all his wondrous works--Zedekiah hopes for God's special interposition, such as was vouchsafed to Hezekiah against Sennacherib (Kg2 19:35-36).
go up from us--rise up from the siege which he sat down to lay (Jer 37:5, Jer 37:11, Margin; Num 16:24, Num 16:27; Kg1 15:19, Margin).
jer 21:4God of Israel--Those "wondrous works" (Jer 21:2) do not belong to you; God is faithful; it is you who forfeit the privileges of the covenant by unfaithfulness. "God will always remain the God of Israel, though He destroy thee and thy people" [CALVIN].
turn back the weapons--I will turn them to a very different use from what you intend them. With them you now fight against the Chaldees "without the walls" (the Jewish defenders being as yet able to sally forth more freely, and defend the fountains outside the walls in the valley under Mount Zion; see Jer 21:13; Jer 19:6-7); but soon ye shall be driven back within the city [MAURER], and "in the midst" of it, I will cause all your arms to be gathered in one place ("I will assemble them," namely, your arms) by the Chaldean conquerors [GROTIUS], who shall slay you with those very arms [MENOCHIUS].
jer 21:5The Jews shall have not merely the Chaldees, but Jehovah Himself in wrath at their provocations, fighting against them. Every word enhances the formidable character of God's opposition, "I myself . . . outstretched hand . . . strong arm (no longer as in Exo 6:6, and in the case of Sennacherib, in your behalf, but) in anger . . . fury . . . great wrath."
jer 21:7the people, and such--rather, explanatory, "the people," namely, "such as are left."
seek their life--content with nothing short of their death; not content with plundering and enslaving them.
smite with . . . sword--This was the fate of Zedekiah's sons and many of the Jewish nobles. Zedekiah himself, though not put to a violent death, died of grief. Compare as to the accurate fulfilment, Jer 34:4; Eze 12:13; Kg2 25:6-7.
jer 21:8"Life," if ye surrender; "death," if ye persist in opposing the Chaldees (compare Deu 30:19). The individuality of Jeremiah's mission from God is shown in that he urges to unconditional surrender; whereas all former prophets had urged the people to oppose their invaders (Isa 7:16; Isa 37:33, Isa 37:35).
jer 21:9(Jer 38:2, Jer 38:17-18).
falleth to--deserts to.
life . . . a prey--proverbial, to make one's escape with life, like a valuable spoil or prey that one carries off; the narrowness of the escape, and the joy felt at it, are included in the idea (Jer 39:18).
jer 21:10set . . . face against--determined to punish (See on Lev 17:10).
jer 21:12house of David--the royal family and all in office about the king. He calls them so, because it was the greater disgrace that they had so degenerated from the piety of their forefather, David; and to repress their glorying in their descent from him, as if they were therefore inviolable; but God will not spare them as apostates.
in the morning--alluding to the time of dispensing justice (Job 24:17; Psa 101:8); but the sense is mainly proverbial, for "with promptness" (Psa 90:14; Psa 143:8). MAURER translates, "every morning."
lest my fury . . . like fire--Already it was kindled, and the decree of God gone forth against the city (Jer 21:4-5), but the king and his house may yet be preserved by repentance and reformation. God urges to righteousness, not as if they can thereby escape punishment wholly, but as the condition of a mitigation of it.
jer 21:13inhabitant of the valley, and rock of the plain--Jerusalem personified; situated for the most part on hills, with valleys at the bottom of them, as the valley of Hinnom, &c.; and beyond the valleys and mountains again, a position most fortified by nature, whence the inhabitants fancied themselves beyond the reach of enemies; but since God is "against" them, their position will avail nothing for them. The "valley" between Mount Zion and Moriah is called TyropÅon. ROBINSON takes, "rock of the plain" as Mount Zion, on which is a level tract of some extent. It is appropriately here referred to, being the site of the royal residence of the "house of David," addressed (Jer 21:12).
jer 21:14fruit of your doings-- (Pro 1:31; Isa 3:10-11).
forest thereof--namely of your city, taken from Jer 21:13. "Forest" refers to the dense mass of houses built of cedar, &c., brought from Lebanon (Jer 22:7; Jer 52:13; Kg2 25:9).
Belonging to an earlier period than the twenty-first chapter, namely, the reigns of Shallum or Jehoahaz, Jehoiakim, and Jeconiah (Jer 22:10, Jer 22:13, Jer 22:20). Jeremiah often groups his prophecies, not by chronological order, but by similarity of subjects; thus Jer 22:3 corresponds to Jer 21:12. GROTIUS thinks that Jeremiah here repeats to Zedekiah what he had announced to that king's predecessors formerly (namely, his brother and brother's son), of a similar bearing, and which had since come to pass; a warning to Zedekiah. Probably, in arranging his prophecies they were grouped for the first time in the present order, designed by the Holy Spirit to set forth the series of kings of Judah, all four alike, failing in "righteousness," followed at last by the "King," a righteous Branch raised unto David, in the house of Judah, "the Lord our righteousness" (Jer 23:6). The unrighteousness of Zedekiah suggested the review of his predecessors' failure in the same respects, and consequent punishment, which ought to have warned him, but did not.