Unrestricted submarine warfare

Unrestricted submarine warfare is a type of naval warfare in which submarines sink merchant ships without warning, as opposed to attacks per prize regulations. While providing the submarine with strongly increased lethality and greater chances of survival against its hunters, it was also considered by many as a substantial breach of the rules of war, especially when employed against neutral country vessels in a war zone.

There have been three major campaigns of unrestricted submarine warfare:

  1. The First Battle of the Atlantic during World War I, waged intermittently by Germany between 1915 and 1918 against Britain and her allies. This warfare was also ostensibly the casus belli for the United States entry into the war in 1917.
  2. The Second Battle of the Atlantic during World War II between 1939 and 1945, waged by Germany, mainly against the Britain and her allies.
  3. The Pacific War during World War II between 1941 and 1945, waged by the United States against Japan.

All the three cases centred around attempts to navally blockade countries (Britain, Japan) heavily depending on merchant shipping to supply their war industries and feed their populations, even though the countries waging the unrestricted submarine warfare were unable to institute a typical naval blockade.


Before WWI

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, neither could it 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 would be under even more pressure to destroy it. 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 and political head of the Navy, supported by senior naval opinion, said it was inconceivable that "...this would ever be done by a civilised power."

It was Fisher who was proved correct, though—as will be shown in the following section—the Germans themselves did not plan for this kind of warfare and went in practice through the stages which Fisher had predicted.

World War I

The evidence suggests that Imperial Germany had not started World War I with an appreciation of the impact on commerce and supply that submarines could have. They had fewer than 30 operational boats, all with small torpedo capacities. At first, merchant ships would be stopped, occupants safely evacuated and then the vessel sunk, usually by gunfire, all following Cruiser Rules. This had little effect and increasingly placed the German submarine—U-boat—at risk from defensive weaponry.

Germany had practical strategic problems. War-weariness affected the German home situation. The best chance of achieving an early advantageous peace with Britain was considered to be the stifling of its trade and imports. Surface ships had not been effective, neither could the Kaiserliche Marine force the British Royal Navy off the seas—the Battle of Jutland had shown this, despite an apparent German victory.

The gamble which was taken was that unrestricted submarine warfare would critically damage Britain before an incensed United States could make a practical impact. Even before the entry of the Unted States into the war, the US had already effectively been on the side of Britain, trading heavily with the United Kingdom, while the UK in turn prevented all attempts at US-German trade via a classical surface ship naval blockade, only ineffectually circumvented by German merchant submarines like the Deutschland. Thus, commercial interests were already in favour of a US that was increasingly assertative in its own strength and its right to trade with whatever nation it desired to, technical neutral or not.

While the success of the submarines was no small blow to British supply lines, the gamble ultimately failed when it drew the United States into the war, and when the introduction of the convoy system cut shipping losses heavily again.

Despite unrestricted warfare in later years, the majority of submarine-caused shipping losses sustained by the Allies were via 'restricted' (Prize Regulations) warfare.

World War II

London Rules on naval warfare

The submarine sinking of merchant ships without warning is in violation of the 1930 First London Naval Treaty, which specifies that "...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..."

However, the London Rules were obsolete before they were signed (though the Kriegsmarine based its Prize Rules on them). The use of disguised guns on auxiliary cruisers increased the risk inherent in stop-and-search rules, but the primary danger came from the wide-spread adoption of radio, which meant that a merchant could call for help as soon as a submarine appeared, even before it could issue its demands. Coupled with the rapidly-growing speed, range, and destructive power of combat aircraft, this technology ensured that complying with these rules would be suicide for any submarine.

Germany in the Atlantic

For the first few weeks of World War II, the German Navy attempted to honour Nazi Germany's treaty obligations, but that attempt was in trouble almost immediately following the sinking of SS Athenia by U 30, and it was abandoned at the end of November or the beginning of December 1939 with the issuing of War Order No. 154.

United States in the Pacific

The United States, from the first day it entered the Pacific War against the Japanese Empire, decided that unrestricted submarine warfare was to be carried out in the Pacific Ocean. This fact was mentioned by American Admiral Chester Nimitz during the post war Nuremberg Trials, in evidence presented at the trial of German Admiral Karl Dönitz on his orders to the U-boat fleet to breach the London Rules. Nimitz's testimony is usually credited as the reason Dönitz's sentence was relatively light (10 years imprisonment) as compared to his co-defendants, seven of whom were sentenced to death by hanging.

Post-WWII concept

Since the introduction of long-range anti-ship missiles after World War II, which are able to destroy a ship from beyond the horizon, the London Rules are universally regarded as entirely void. It is indicative that despite the rules being used in the indictment of Admiral Karl Dönitz, and although he was found guilty of breaching the 1936 Naval Protocol, his sentence was not assessed on the ground of his breaches of the international law of submarine warfare at the Nuremberg Trials.

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