They understood the risk they were running, that this would probably bring the United States into the war, but calculated that by the time American participation became effective the war would have been won. It was, as a German statesman put it, Germany’s last card; ‘and if it is not trumps, we are lost for centuries’. He was not far wrong. In 1914 few navies had understood the potential of the submarine. The range of the first petrol-driven models made them suitable only for coastal defence, and even when, shortly before the war, submarines were equipped with diesel-driven engines, they remained basically ‘submersibles’—highly vulnerable on the surface and with a very limited submerged capacity.
Their potential lethality was demonstrated within weeks of the outbreak of war when, as we have seen, a German submarine had sunk three unwary British cruisers in the Channel. But warships were regarded as fair game. Unarmed merchantmen were not. Over some three centuries of trade warfare the maritime powers of Europe had evolved elaborate rules for the treatment of merchant vessels on the high seas in wartime. Belligerents had the right to stop and search them for ‘contraband’,— that is, materials of war. If any was found, the vessel had to be escorted to the nearest port, where a ‘prize court’ would adjudicate whether the cargo was contraband or not, and confiscate it if it was.
If for any reason this was not possible, the vessel might be destroyed, but only after the passengers and crew had been put in a place of safety. For a submarine, none of this was possible. They had no space either for a spare crew to man captured vessels or to accommodate their prisoners. If they surfaced to give warning of attack, they were vulnerable to any armaments their victim might be carrying, and to having their position instantly revealed by his radio; but to sink the vessel without warning and without saving her crew was, in the view of pre-war naval strategists, ‘unthinkable’.
None the less, blockade had always been central to the conduct of war between maritime powers, and the advent of industrialization had made it more central than ever. In wars between agrarian societies, blockade could destroy only trade and with it the wealth that enabled states to carry on the war. Populations could still feed themselves. But blockade of industrialized societies, especially ones so highly urbanized as Britain and Germany, would not only interrupt trade and so (it was believed) create financial chaos, but destroy industries by depriving them of imported raw materials, to say nothing of starving urban populations by depriving them of imported foodstuffs. This was the nightmare that had haunted pre-war British planners and publicists when they contemplated the implications of losing ‘command of the sea’; and this was the weapon by which the British Admiralty had hoped to achieve victory over Germany without the need for any major military commitment to the Continent.