Jeremiah 15:12 - Matthew Poole's English Annotations on the Holy Bible

There is a great variety among interpreters as to this verse also, some interpreting this as a prophecy that none should break the prophet, whom God would make as the northern iron and steel, which was the hardest of all iron, the Chalybes (from whom steel had its name Chalybs) being northern people, and the most famous of any then known in the world for tempering iron to make it hard and tough; others interpreting it, as denying that there should ever be an agreement betwixt the Jews and the Chaldeans: but to me the words of the next verse seem to give us the sense, that the Jews should certainly be overrun and conquered by the Babylonians; for as the northern iron and steel is the hardest, and no iron could break that, so God having edged and hardened their enemies the Chaldeans, all their opposition to them would signify nothing.