Tonnage war

A tonnage war is a military strategy aimed at merchant shipping. The premise is that the enemy has only a number of ships, and a finite capacity to build replacements for them. The concept was made famous by U-boat commander Karl Dönitz, who wrote: 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 primarily aim at a relatively narrow set of goals. For example, a traditional practice of the Royal Navy during wars between Britain and France was blockade. By concentrating available naval units forces near the major French ports, the Royal Navy was usually able to strangle French trade and create significant economic difficulties. Similarly, the enemy may focus on ships carrying strategically vital cargos such as hemp and timber or, in modern times, oil and iron. Alternatively, the aim may be to target ships carrying particularly valuable cargos such as treasure or munitions, and ships carrying less critical cargoes or steaming in ballast are assigned a lower priority.

In general, these relatively narrow strategies require that the attacker establish substantial control over a particular area. For example, 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 to make a challenge. Similarly, during the Siege of Malta (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 into grave danger.

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