Jeremiah 21:4

Thus saith the Lord God of Israel
Who had been, still was, and would be, Israel's God, even the God of such who are Israelites indeed; though he should, as he would, give up the present generation to ruin and destruction; they having by their sins forfeited his care and protection of them; and therefore it was in vain to hope for it from this character which they bore: behold, I will turn back the weapons of war that [are] in your hands;
so that they should do no hurt to the enemy, but recoil upon themselves. The meaning is, that they should be useless and unserviceable; that they should neither be defensive to them, nor offensive to their enemies; but rather hurtful to themselves. It seems to suggest, as if they should fall out with one another; and, like the Midianites, turn their swords upon one another, and destroy each other: wherewith ye fight against the king of Babylon, and [against] the
Chaldeans which besiege you without the walls;
by shooting arrows at them from within the city; or by sallying out unto them with sword in hand: this, shows that the Chaldean army, under the command of the king of Babylon, was now without the walls of Jerusalem besieging it: and I will assemble them into the midst of this city;
either the weapons of war, as Jarchi and others; which the Chaldeans, breaking into the city, should cause to be brought in to them in the middle of the city, and there slay them with them: or rather the Chaldeans, as Kimchi; who, though now without the walls, and which the Jews thought a sufficient security for them; yet should not be long there, but the walls would be broken down, and they should enter the city, and rendezvous their whole army in the midst of it.