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:"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 he sink ships more rapidly than the defender can replace them.
During World War II, three tonnage wars were fought. The largest and best known of them was Nazi Germany's U-boat campaign, aimed mainly against the United Kingdom. Less well-known campaigns were waged by Allied forces in the Mediterranean and Pacific theaters, neither of them deliberately planned as a tonnage war in the way that German U-boat campaign was, but both having that effect— and both were also very successful.
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Anti-submarine warfare (ASW, or in the older form A/S) is a branch of underwater warfare that uses surface warships, aircraft, submarines, or other platforms, to find, track, and deter, damage, or destroy enemy submarines. Such operations are typically carried out to protect friendly shipping and coastal facilities from submarine attacks and to overcome blockades. Successful ASW operations typically involved a combination of sensor and weapon technologies, along with effective deployment strategies and sufficiently trained personnel.
Unrestricted submarine warfare is a type of naval warfare in which submarines sink merchant ships such as freighters and tankers without warning, as opposed to attacks per prize rules (also known as "cruiser rules") that call for warships to search merchantmen and place crews in "a place of safety" (for which lifeboats do not qualify, except under particular circumstances) before sinking them, unless the ship shows "persistent refusal to stop ... or active resistance to visit or search".
Allied submarines were used extensively during the Pacific War and were a key contributor to the defeat of the Empire of Japan. During the war, submarines of the United States Navy were responsible for 56% of Japan's merchant marine losses; other Allied navies added to the toll. The war against shipping was the single most decisive factor in the collapse of the Japanese economy. Allied submarines also sank a large number of Imperial Japanese Army (IJA) troop transports, killing many thousands of Japanese soldiers and hampering the deployment of IJA reinforcements during the battles on the Pacific islands.