John Calvin Commentary Nahum 3:12

"All thy fortresses shall be [like] fig-trees with the first-ripe figs: if they be shaken, they fall into the mouth of the eater." — Nahum 3:12 (ASV)

The Prophet here declares that the strongholds of the Assyrians would be of no use to them; whether they trusted in the number of their men, or in their walls, or in other defenses, they would be disappointed; for all things, he says, will fall on their own, even without being heavily attacked. He employs a very fitting comparison: Your fortifications, he says, which you think are very strong, will be like figs; for when the fruit is ripe, and anyone comes to the tree, as soon as he touches it or any of the branches, the figs will fall off on their own.

We indeed know that there is not much firmness in that fruit; when it is ripe, it immediately falls to the ground, or if it hangs on the branches, a very slight shaking will bring it down. We now understand the Prophet's intention.

From this, a useful doctrine may be derived: whatever strength people may seek for themselves from various sources, it will completely vanish; for neither forts, nor towers, nor ramparts, nor troops of men, nor any kind of contrivances, will be of any use; and even if no one were to rise against them, they would still fall on their own.