Nahum 2:6

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 to have been a most rapid river,) the Prophet ridicules the confidence of the Ninevites, who thought that the access of enemies could be wholly prevented in that part where the Tigris flowed. The gates then of the rivers are opened; that is, your river shall not prevent your enemies from breaking through and penetrating into your city.

We hence see, that the Prophet removes all the hindrances which might have seemed available to keep off enemies; and he did so, not so much for the sake of Nineveh as for the sake of his chosen people, that the Israelites and Jews might know, that that city was no less in the power of God than any other; for God can no less easily pass through rivers than 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 approach of enemies; for all the fortresses will melt away, and that of themselves, as though they were walls of paper, and the stones, as though they were water. He afterwards adds —