Jamieson, Fausset and Brown on Jeremiah 21:4

4. God of Israel--Those "wondrous works" (Jr 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 Jr 21:13; Jr 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].