From the outset of the Great War, it was obvious that a major factor in the war was the need of the British to import huge quantities of material and foodstuffs. Being an island nation, inevitably, this meant transportation by sea. Even in peacetime, these imports amounted to around 50% of the nation's needs. To carry this huge amount of material the British Merchant Navy had around 20 million tons of shipping. Added to this were the goods carried by the merchant ships of other nations; significant among which were the ships of the United States and Italy, neither of which sided in the war with the Central Powers of Germany and Austro-Hungary (later, including Turkey and Bulgaria).
Of course, the Central Powers were well aware of this Achilles Heel in the British strategy for war, and plans were already in place on the eve of the war to deprive the British of the essential needs to prosecute the war on the Western Front and elsewhere. The German term for this kind of action against merchant shipping was 'Handelskrieg' = Trade Warfare. The Central Powers were also well aware that attacks without warning on merchant shipping were considered by most maritime nations as war-crimes, and that most of them were signatories to international charters to this effect, including Germany itself.
The plan of the Central Powers for action at sea, at the outbreak of war in August 1914, was to use networks of fast warships and auxiliary commerce raiders disguised as merchant ships. These ships would scour the Seven Seas in search of the British Merchant Navy ships and sink, or capture, them as and when the opportunity arose whilst, in principle, obeying the rules of engagement; the so-called 'Prize Rules'.
Initially, this mode of action had some success, but was soon largely neutralised by the Royal Navy with considerable losses to the German ships. Additionally, the British marine blockade of Germany was far more effective than the Germans had anticipated. The North Sea virtually became a British 'lake'.
By the autumn of 1914, the thoughts of the leaders of the Central Powers turned to more drastic and illegal methods.
Countermeasures at sea
Up to the outbreak of the war, few military strategists had given thought to the use of submarines against merchant vessels, and even less against those of the neutral countries. The whole idea was denounced by some influential politicians as 'barbaric'. However, as the earlier Handelskrieg measures showed no significant results, the German Naval staff led by the German Naval Chief of Staff, Admiral Hugo van Pohl, and the Naval Minister, Grand Admiral Alfred von Tirpitz, began to support more 'unrestricted warfare' mainly using submarines. They anticipated that rapid results in the Handelskrieg were obtainable if unrestricted attacks by their submarines were pushed home with sufficient vigour and ruthlessness.
Doubts were expressed by the German chancellor, Theobald von Bethmann Hollweg, and his advisers, that there were sufficient submarines in the German fleet (it had 20) to carry out such an ambitious and dangerous strategy. The Chancellor provided figures showing that at least 200 would be needed to neutralise the British Navy and establish an effective counter blockade. A figure that events proved to be remarkably prescient.
There is some evidence that the fiasco of the German High Fleet in the mutually unsuccessful naval action of the British and German battle cruisers at Dogger Bank, on the 24th January 1915, much influenced the eventual decision by the Kaiser, Wilhelm II, to authorise submarine warfare as the major means of pursuing the Handelskrieg.
On the 4th February 1915, the German Navy declared all the seas around Britain and Ireland to be a 'war zone', and that all Allied merchant ships would be sunk 'without warning'. Obviously, this was in contravention of the international charters of the sea. Despite strong protests by the neutral nations, by the end of February 1915, 16 German U-boats (Unterseeboot - it was Winston Churchill, then First Lord of the Admiralty, who coined the name U-boat) were seeking out merchant ships in the sea-lanes and approaches around the British and Irish coasts.
Success was quickly achieved, with 85,000 tons of Allied shipping being sunk in March 1915 alone.
The British response
The British responded aggressively, creating the so-called Dover Barrage that effectively prevented free access to the English Channel. However, the German Navy now had access to ports on the Belgian coast, and Allied losses rose to a total of 300,000 tons from May to July 1915 for a total cost, to-date, of seven U-boats.
In view of the huge tonnage of shipping available to the Allies, these losses, though painful, were not really significant in terms of affecting the prosecution of the war. Meanwhile, the neutral nations again protested vehemently against the unrestricted submarine campaign; particularly after the loss of the U.S. passenger liner the Lusitania, within sight of the coast of Ireland.
Following the furore about the sinking of the Lusitania, inevitably, political considerations caused a realignment of German strategy, and attacks on passenger ships were formally forbidden by the Kaiser himself.
Despite these restrictions, and the British counter measures, including the deployment of over 2000 Armed Auxiliary Ships and the decoy vessels (the 193 so-called 'Q ships'), by the end of September the U-boats had accounted for over 700,000 tons of Allied shipping (mostly British) for the loss of 12 of the longer-range ocean-going U-boats.
Again, the German naval offensive strategy was rethought, with five U-boats being sent to the Mediterranean along with several of the smaller, less sophisticated, submarines; the so-called UB- and UC-boats. These submarines jointly had considerable success. Another detachment of five U-boats, which were operational in the Black Sea, had much slimmer pickings.
During 1915, the production of U-boats in the German shipyards quickly accelerated. By the end of the year the fleet had doubled, far out-weighing the losses at sea.
Frustrated by their inability to seriously dent the tonnage of available Allied ships, in March 1916, the German Navy, under a new High Fleet C-in-C - Admiral Reinhardt Scheer - again undertook an unrestricted campaign. But this was brought short by the sinking of the U.S. vessel Sussex and the restrictions were again imposed.
In the Mediterranean, in 1916, the U-boat war continued to be productive, with over 250 Allied ships being sunk in the second half of the year; there were only minor losses to the U-boats.
The political struggle by the German chancellor to control the U-boat campaign continued in 1916, and a serious effort was made by the chancellor to try to find some accommodation with the USA. The lack of international support for any kind of cease-fire meant that these advances got no-where. The now U-boat dominated Handelskrieg continued, with some restrictions, achieving monthly sinkings of around 300,000 tons into 1917.
The total number of operational U-boats also continued to rise and, at the beginning of 1917, exceeded 100. Having failed to obtain the Allies acceptance of a Peace Offer, both the German political and military camps approved the return of an unrestricted U-boat based Handelskreig from the 1st February 1917.
The resulting sudden resurgence in the sinking of Allied and neutral shipping, which soared to exceed 800,000 tons in April 1917 - with the loss of only two German U-boats - brought to a head the British Navy's long-going indecision over the deployment of a convoy system on the major shipping routes. On the 27th April 1917, the convoy system was formally introduced and shipping on the Far-East route was redirected around Africa, via the Cape of Good-Hope, avoiding the vulnerable waters around the Suez Canal. The entry into the war by the U.S.A. in the same month, further galvanised the introduction of across-the-Atlantic convoys.
The loss of Allied shipping tonnage over the next three months - May to August 1917 - although less severe, was not very encouraging. Also, the increase in operational U-boat numbers (150+) continued, with few losses. But the Allied shipping losses did not have an evident effect on British arms production and social life to the extent that the Germans had anticipated. Also, there was now clear evidence that the ships sailing in convoys were, indeed, far less vulnerable to U-boat attack.
This stalemated condition continued into 1918, with German ambitions for ever mounting U-boat production continuing right up to the Armistice, when over 250 were still on order. But, overall, by the Spring of 1918, the tide was turning, with the convoy system, and its associated land- and sea-based U-boat counter-measures, taking an ever increasing toll: 14 U-boats in May 1918 and 150 since the war began. Further difficulties for the U-boat commanders arose from the loss of experienced crews and the overall decline of the efficacy of the German war machine.
The final closure of the Handelskreig occurred in October 1918, when the German Naval Command decided to put all available naval units into a final suicidal attack on the British Grand Fleet. But this was foiled by the mutiny of the dissatisfied crews of the German surface ships based at Kiel, followed by the collapse of the German army and State, leaving the remaining scattered U-boats to return to Germany and surrender to the triumphant Allies.