This was practically the case with Belgium when it was a Spanish and an Austrian province.
The Dutch, in 1648, as a condition of peace after a successful war, exacted that the Scheldt should be closed to sea commerce.

This closed the harbor of Antwerp and transferred the sea trade of Belgium to Holland.

The Spanish Netherlands ceased to be a sea power.
Numerous and deep harbors are a source of strength and wealth, and doubly so if they are the outlets of navigable streams, which facilitate the concentration in them of a country's internal trade; but by their very accessibility they become a source of weakness in war, if not properly defended.

The Dutch in 1667 found little difficulty in ascending the Thames and burning a large fraction of the English navy within sight of London; whereas a few years later the combined fleets of England and France, when attempting a landing in Holland, were foiled by the difficulties of the coast as much as by the valor of the Dutch fleet.

In 1778 the harbor of New York, and with it undisputed control of the Hudson River, would have been lost to the English, who were caught at disadvantage, but for the hesitancy of the French admiral.