Tonnage war

A tonnage war is a military strategy aimed at merchant shipping. The premise is that an enemy has only a finite 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, an 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 early part of World War II, Axis air forces controlled the Mediterranean and were able to prevent Allied ships from reaching Malta with supplies and were almost successful in reducing the island fortress.

A tonnage war, however, is a very broad strategy, and does not require that the attacker establish control over any particular area, merely that he is able to sink ships more rapidly than the defender can replace them.

During the Second World War, three tonnage wars were fought. The largest and best known of them was Dönitz's U-boat campaign, aimed mainly against the United Kingdom. Although the primary venue for the campaign was the North Atlantic, Dönitz sent U-boats and surface raiders to all corners of the globe in search of the most efficient way to sink the maximum number of ships at minimum cost. Despite an enormous allocation of resources, the U-boats were able to reduce the total shipping available to the Allies in only one of the five years of the war, 1942. Before that year, too few submarines were available, and after May 1943 the mass production of liberty ships in the USA and the ever-increasing technical sophistication of the Allied anti-submarine forces saw the ratio between sinkings and new construction change radically. A key factor in the British anti-submarine effort was the success of the codebreakers at Bletchley Park in reading messages encrypted using the German Enigma machine. To prevent the Germans from guessing that they were had broken Enigma, the British planted a false story about a special infrared camera being used to locate U-boats. The British were subsequently delighted to learn that the Germans responded by developing a special paint for submarines that exactly duplicated the optical properties of seawater.

Less well-known but almost equally significant were two campaigns by Allied forces, neither of them deliberately planned as a tonnage war in the way that Dönitz's U-boat campaign was, but both having that effect—and paradoxically, both were highly successful. The first of these was the Allied campaign against Axis shipping (mostly Italian) from Europe to North Africa. British submarines based in Malta and the aircraft of several Allied air forces, in conjunction with British and Commonwealth surface ships, succeeded in reducing shipments of essential military supplies to Axis forces under Rommel to the point where the German commander was unable to fight effectively. By the close of the campaign, Italy had very few merchant ships left.

In the early years of the Pacific War, the submarines of the US Navy were allocated a great variety of tasks and unable to achieve any of them effectively, particularly given major technical problems with the torpedoes they were armed with. From about the middle of 1943, however, substantial numbers of American submarines were tasked with disrupting Japanese trade, in particular, with cutting off the flow of oil and other vital materials from the occupied territories of South-east Asia. This, too, became a tonnage war, with rapidly building results, and by mid to late 1944 Allied submarines and aircraft were experiencing difficulty in finding targets large enough to justify expending a torpedo on. The Japanese merchant navy was all but wiped out, and despite desperate measures to make do without strategic materials, the war economy ground to a virtual standstill.