John Calvin's Bible Commentary
Jeremiah 51:30
The Prophet shows here, as by the finger, the manner of the destruction of Babylon, such as it is described by heathen authors. He then says, that the valiant men of Babylon, even those who had been chosen to defend the city, ceased to fight For the city was taken rather by craft than by open force; for after a long siege, Cyrus was laughed to scorn by the Babylonians; then they securely held a feast. In the meantime two eunuchs of Belshazzar passed over to Cyrus; for; as Xenophon relates, the tyrant had slain the son of one, and by way of disgrace castrated the other. Hence, then, it was that they revolted from him; and Cyrus was instructed by them how he could take the city. The fords were dried-up, when Belshazzar suspected no such thing, and in the night he heard that the city was taken. Daniel gives a clearer description; for he says that there was held a stated feast, and that the hand of a writer appeared on the wall, and that the king, being frightened, had heard from Daniel that the end of his kingdom was near at hand, and that the city was taken that very night. (Daniel 5:25.) hence the Prophet says now that the valiant men desisted, so that they did not fight. He indeed speaks of what was future, but, we know what was the manner of the prophets, for they related what was to come as though it had already taken place.
He afterwards adds, that they sat down in their fortresses, for the city was not taken by storm — there was no fighting; but the forces passed silently through the fords, and the soldiers entered into the middle of the city; the king was slain together with all his satraps, and then all parts of the city were taken possession of. We now, then, see that the Spirit of God spoke by the mouth of Jeremiah, as of a thing that had already taken place.
He then adds, that their valor had failed or languished, even because terror stupefied them when they heard that the city was taken. So also true became what is added, that they became women, that they were like women as to courage, for no one dared to oppose the conquerors. Fighting might have still been carried on by so large a multitude, yea, they might have engaged with their enemies in hundred or in thousand of the streets of the city, for it would have been easy in the night to distress them: but the Prophet says, that they all became women as to courage. At last, he adds, that that burnt by enemies were the palaces, and that the bars of the gates were broken; for no one dared to summon to arms after it was heard that the city was taken. It follows, —