John Calvin on Jeremiah 50:43

The Prophet means by these words, that as soon as the report of war reached the Chaldeans, they would be so disheartened through fear as to become like a conquered people. As they had subjected to themselves many nations, they had acquired the name of being a warlike people; but the Prophet declares here that they would have no courage, and that therefore there would be no need of much valor to attack them, as they would of themselves give way and flee. The sum of what is said is, that the Persians and the Medes would gain the victory before they fought, for there would be no need of an attack, as their enemies would flee as being without any courage.

The Prophet at the same time intimates that in God’s hand are the hearts of men, as I have often said, so that they who seem to excel in great boldness, melt as wax in a moment. For no doubt the Chaldeans were not wanting in courage to fight until God had rendered them effeminate, so that they took to flight through fear as soon as they heard the report respecting their enemies. It is, indeed, true that this was not immediately the case, for we know that they had long sustained a siege, and that Belshazzar was slain in the night, while they were securely and joyfully feasting as in the greatest quietness and peace; but they were at length taken, so that they had neither wisdom nor confidence; for the king and his princes were slain, and the city was in a moment taken, as though all the men were turned into logs of wood or into statues of stone. It follows,—

Jeremiah 50:44

44. Behold, he shall come up like a lion from the swelling of Jordan unto the habitation of the strong: but I will make them suddenly run away from her: and who is a chosen man, that I may appoint over her? for who is like me? and who will appoint me the time? and who is that shepherd that will stand before me?

44. Ecce tanquam leo ascendet (ascondens) a tumore Jordanis (ab altitudine, vel, elevatione, proprie, גאון etiam significat metaphorice superbiam) ad habitaculum forte, quum quiescere fecero, (vel, postquam irruptionem fecero,) currere faciam eos ab ipsa; et quis electus quem super eam praeficiam? Quis enim similis mei? et quis contestabitur mecum? et quis ille pastor qui consistat coram me (vel, ad faciem meam)?