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a conventional term referring to combat waged by submarines.
Submarine warfare was used extensively in World War I and World War II. In World War I the losses to the merchant fleets of the warring states from submarine attacks totaled 14 million gross registered tons and 192 warships. Germany used submarines not only against warships but also, in defiance of the norms of international law, against the merchant ships of enemy and neutral states. After the war, the question of prohibiting submarine warfare was discussed at conferences in Washington in 1921–22 and London in 1930 and 1936. At the Washington Conference of 1921–22, Great Britain presented a proposal to ban the use of submarines. It was not adopted, being opposed, in particular, by the United States.
In 1936 the United States, Great Britain (with its dominions and India), France, Italy, and Japan signed the London Protocol, which contained rules for submarine actions with respect to merchant ships in wartime. The USSR joined the protocol in 1937 as did a number of other states, such as Belgium, the Netherlands, the Scandinavian countries, and Germany. According to the protocol, submarines in actions related to merchant vessels were to be guided by the norms of international law in the same way as surface warships. If a merchant ship stubbornly refused to halt after an appropriate warning to do so or if the ship offered resistance to being inspected or searched, only then could it be sunk or prevented from traveling further. However, stipulation was made that the passengers, crew, and ship documents be delivered first to a safe place. In World War II losses to the merchant fleet from submarines totaled more than 22 million gross registered tons and about 400 warships. Fascist Germany and Japan, which had signed the agreements, violated them many times.