John Calvin Commentary Jeremiah 50:43

"The king of Babylon hath heard the tidings of them, and his hands wax feeble: anguish hath taken hold of him, [and] pangs as of a woman in travail." — Jeremiah 50:43 (ASV)

The Prophet means by these words that as soon as the report of war reached the Chaldeans, they would be so disheartened by fear that they would become like a conquered people.

Since they had subjected many nations to themselves, they had acquired the reputation of being a warlike people. But the Prophet declares here that they would have no courage. Therefore, there would be no need for much valor to attack them, as they would give way and flee on their own.

In summary, the Persians and the Medes would gain the victory before they fought, because there would be no need for an attack, as their enemies would flee, being without any courage.

The Prophet at the same time intimates that the hearts of men are in God’s hand, as I have often said, so that those who seem to excel in great boldness melt like wax in a moment.

For undoubtedly, the Chaldeans did not lack courage to fight until God had broken their manly spirit, causing them to flee in fear as soon as they heard the report about their enemies.

It is true, indeed, that this did not happen immediately. We know that they endured a long siege, and Belshazzar was slain in the night while they were feasting securely and joyfully, as if in the greatest quietness and peace.

But they were eventually captured, and as a result, they had neither wisdom nor confidence. For the king and his princes were slain, and the city was captured in a moment, as though all the men had turned into logs of wood or statues of stone.