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 many 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 . World War II . 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 - boat s 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 . The U - boats campaign was very successful especially in the two happy periods ( in 1940 and in 1942 ) , and was able to reduce the total shipping available to the Allies up to a breakage point until 1943 , when the tide of war was turning against Germany . On the other side , an important 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 . Even more important was the introduction of shipborne direction finder s ( known as HF / DF , or " Huff Duff " ) . Less well - known 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 both were also very 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 submarine s of the US Navy were allocated a great variety of tasks and were unable to achieve any of them effectively , particularly given major technical problems with the Mark 14 torpedo es they were armed with . From about the middle of 1943 , however , substantial numbers of American submarines were tasked with disrupting Japan ese 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 .