John Calvin Commentary Nahum 2:6

"The gates of the rivers are opened, and the palace is dissolved." — Nahum 2:6 (ASV)

By the gates of the rivers, the Prophet means that part of the city which was most fortified by the river Tigris; for the Tigris flowed close by the city. As, then, the Tigris was like the strongest defense (for we know it was a very rapid river), the Prophet ridicules the confidence of the Ninevites, who thought that the access of enemies could be entirely prevented in that part where the Tigris flowed. The gates then of the rivers are opened; that is, your river will not prevent your enemies from breaking through and penetrating into your city.

We therefore see that the Prophet removes all the obstacles that might have seemed capable of keeping enemies away; and He did this not so much for Nineveh's sake as for the sake of His chosen people, so that the Israelites and Jews might know that that city was no less in God's power than any other; for God can pass through rivers no less easily than He can go along the plain, where there is no obstacle.

We now see why the Prophet says that the gates of the rivers were opened: and then he adds, The palace is dissolved; that is, there will be no impediment to prevent the enemies' approach, for all the fortresses will melt away by themselves, as though they were walls of paper, and the stones as though they were water. He afterwards adds—