John Calvin Commentary Isaiah 13:7

"Therefore shall all hands be feeble, and every heart of man shall melt:" — Isaiah 13:7 (ASV)

Therefore all hands shall be weakened. He shows that the power of the Lord to destroy the inhabitants of Babylon will be so great, that they will have no means of withstanding his anger. Though they stood high in wealth and in power, yet their hearts would be so faint, and their hands so weak, that they would have neither disposition nor ability to resist.

And so he indirectly ridicules the cruelty which boiled in the hearts of the Babylonians. For it is in the power of God to soften hearts, and to crush, loosen, or enfeeble hands or arms, so that suddenly all their courage will fall down, and all their strength will vanish away.

When the heart quakes, what will be the use of fortifications, or armies, or wealth, or bulwarks? What good is a well-stocked workshop without a workman? We see this every day exemplified in those to whom, in other respects, the Lord had given large resources. Therefore, we see how vain is that confidence which we place in outward resources; for they would be of no use to us if the Lord should strike our hearts with any alarm.