There have been three major campaigns of unrestricted submarine warfare:
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.
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.
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.
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.