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Customary rules were originally laid down in the days of sailing ships. These were supplemented by various international agreements including the Declaration of Paris (1856) and the Hague Conventions (1899 and 1907) and other naval agreements during the 20th century. Although these rules are still part of the laws of war, changes in technology like the radio and the submarine made them redundant between belligerents during World War I and World War II. Still, the Nuremberg Tribunal found that these rules were still applicable to neutral merchant shipping.
Prize rules state that passenger ships may not be sunk, crews of merchant ships must be placed in safety before their ships may be sunk (life boats are not considered a place of safety unless close to land); and only warships and merchant ships that are a threat to the attacker may be sunk without warning.
Declaration of Paris, 1856
In 1856 and afterward, numerous states, including the United Kingdom, Austria-Hungary, France, Prussia, Russia, Sardinia and the Ottoman Empire, ratified the Declaration of Paris. It regulated the relationship between neutral and belligerent and shipping on the high seas when the signatories were fighting each other, but not when fighting non-signatory nations. The United States withheld its formal adherence in 1857.
Part IV, Art. 22 of the Treaty for the Limitation and Reduction of Naval Armaments, relates to submarine warfare. It states as follows:
- In their action with regard to merchant ships, submarines must conform to the rules of international law to which surface vessels are subject.
- In particular, except in the case of persistent refusal to stop on being duly summoned, or of active resistance to visit or search, a warship, whether surface vessel or submarine, may not sink or render incapable of navigation a merchant vessel without having first placed passengers, crew and ship's papers in a place of safety. For this purpose the ship's boats are not regarded as a place of safety unless the safety of the passengers and crew is assured, in the existing sea and weather conditions, by the proximity of land, or the presence of another vessel which is in a position to take them on board.
All sides signed treaties (the Hague Conventions of 1899 and 1907) subscribing to rules of prize warfare before World War I and they were also in effect during World War II. During 1914-1917, Germany adhered to the prize rules until it declared unrestricted submarine warfare. During World War II, Germany adhered to the prize rules for the first 2 months of the conflict in 1939 before turning again to unrestricted submarine warfare. The United States Navy applied unrestricted submarine warfare during the Pacific War. In addition, the Royal Navy and the Soviet Navy employed unrestricted submarine warfare during World War II against Germany in the Skagerrak and the Baltic Sea, respectively.
In 1912, British Admiral Sir John "Jackie" Fisher, by then a retired First Sea Lord, presented a paper to the Cabinet. He developed the argument that submarines would find adherence to prize rules impossible for practical reasons: a submarine could not capture a merchant ship, for it would have no spare manpower to deliver the prize to a neutral port; it could not take survivors or prisoners, for lack of space: "there is nothing a submarine can do except sink her capture." If a merchant ship were armed, as was permitted by a conference in London in 1912, then a submarine was under more pressure to destroy a ship. He asked, "What if the Germans were to use submarines against commerce without restriction?"
This last comment was thought to be unsupportable. Winston Churchill, then First Lord of the Admiralty, supported by senior naval opinion, said it was inconceivable that "this would ever be done by a civilised power." However, it was Fisher who was proven correct.
The treaties are still in effect today.