Tonnage war

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A tonnage war is a military strategy aimed at merchant shipping. The premise is that the enemy has a finite number of ships and a finite capacity to build replacements. The concept was made famous by German Grand Admiral Karl Dönitz, who wrote:^[1]

"The shipping of the enemy powers is one great whole. It is therefore in this connection immaterial where a ship is sunk—it must still in the final analysis be replaced by a new ship".

Most anti-shipping strategies have had a relatively narrow set of goals. A traditional practice of the Royal Navy during wars between Britain and France was the blockade. By concentrating naval forces near large French ports, the Royal Navy was usually able to impede French trade to the point of creating significant economic difficulties. The opponent may focus on ships carrying strategically vital cargoes such as hemp and timber or, in modern times, oil and iron. The aim might be to attack ships carrying particularly valuable cargoes such as treasure or munitions and ships carrying less important cargoes or steaming in ballast are at first ignored.

These narrow strategies require the attacker to establish substantial control over an area. The British blockades of France were only possible so long as the Royal Navy retained the ability to defeat any French squadron venturing out from port. During the Siege of Malta in World War II, Axis air forces had air superiority and were able to prevent many Allied ships from reaching Malta with supplies, putting the island fortress in grave danger.

A tonnage war is a broad strategy. As a form of attrition warfare, it does not require the attacker to establish control over an area, merely that they sink ships more rapidly than the defender can replace them.