In February 1916 Germany announced that from 1 March its U-boats would sink defensively armed British merchant ships without warning. Germany had, under pressure from the USA, abandoned unrestricted submarine warfare in September 1915. The Germans claimed that British defensively armed merchantmen had been ordered to attack U-boats before being attacked themselves, so could not be regarded as acting defensively. The Admiralty quickly disproved this accusation by publishing the actual orders.
The German position on the issue of unrestricted submarine warfare is described by Paul Halpern as being ‘somewhat confusing’. The naval and military commanders wanted to pursue unrestricted submarine warfare but Kaiser Wilhelm and Chancellor Theobald von Bethmann Hollweg were concerned about US opinion. The US government opposed the treatment of armed merchant ship as warships.
Following a conference on 4 March the Kaiser accepted that unrestricted warfare was necessary, with a likely start date of 1 April. Until then, Bethmann Hollweg should attempt to persuade the Americans to accept the German view. In the interim U-boats were authorised to sink enemy merchant ships in the war zone and armed ones outside it. Passenger liners, whether armed or not, could not be attacked anywhere by submerged U-boats. Halpern describes this as ‘sharpened’ rather than unrestricted submarine warfare.
Admiral Alfred von Tirpitz, the architect of the German battlefleet had not been invited to the 4 March meeting. He resigned as State Secretary of the Imperial Naval Office, a job mainly concerned with administration and naval building, soon afterwards.
The Germans had 52 U-boats in March 1916, compared with 29 or 30 when they began unrestricted submarine warfare a year before. Of these 16 were in the North Sea, 20 small and short ranged ones with the Flanders Flotilla, 4 in the Baltic, 7 in the Adriatic and 5 at Istanbul. It was expected that another 38 would be completed by August.
Several Dutch merchant ships were sunk in March, including the 13,911 liner Tubantia, the largest neutral ship sunk during the war. The Dutch were angered by these losses, but the attitude of the USA was far more important to the Germans.
On the afternoon of 24 March the 1,353 ton French packet Sussex was torpedoed and badly damaged by UB29 (Oberleutnant Herbert Pustkuchen) whilst making her regular run from Folkestone to Dieppe with over 325 passengers. The Germans claimed at first that she had hit a mine, but fragments of a torpedo were found after she had been towed into Boulogne.
The 50 dead included some of the 25 Americans on board. Pustkuchen claimed that he assumed from her crowded decks that she was a troopship. British troopships then crossed at night between Folkestone and Boulogne. The Germans may not have known that and Pustkuchen, with the very limited view offered by a periscope, may have genuinely thought that he was firing at a troopship. His action, however, created a major diplomatic incident with the USA.
On 19 April US President Woodrow Wilson told Congress that that unless the Germans abandoned ‘their present method of warfare against passenger and freight-carrying vessels, the Government have no choice but to sever diplomatic relations with the German Government altogether.’
On 20 April the Germans agreed to stop sinking merchant ships without warning. Four days later Admiral Reinhard Scheer, the recently appointed C.-in-C. of the High Seas Fleet, recalled all his U-boats from the North Sea on the grounds that there was no point in sending boats on dangerous missions when their actions were so restricted. This left only the short ranged boats of the Flanders Flotilla operating against Allied shipping in British waters.
The following table shows Allied shipping losses to U-boats since the end of the period of unrestricted submarine warfare. Sinkings rose in April and May and then fell again, especially outside the Mediterranean, to which several U-boats were transferred in September 1915 so that they could raid Allied commerce in an area where the risk of killing Americans was reduced, although not eliminated. On 7 November U38, a German boat that was flying Austro-Hungarian colours because Germany and Italy were not yet at war, sank the Italian liner Ancona, killing over 200 people, including about 20 Americans.
|Merchant shipping Losses to U-boats|
|Month||Ships||Gross tons||Ships||Gross tons|
Source: P. G. Halpern, A Naval History of World War I, (London: UCL Press, 1994), p. 308.
The Germans thought that unrestricted submarine warfare was their best chance of forcing Britain out of the war. However, it also risked bringing the USA into the war on the Allied side. In 1916 they had too few U-boats to achieve the former but pursuing this strategy could still cause the latter.
 Naval Staff Monograph (Historical) 1926 vol. xv, Home Waters part vi, October 1915 to May 1916. p. 94.
 P. G. Halpern, A Naval History of World War I (London: UCL Press, 1994), p. 305.
 Ibid., p. 306.
 Ibid., p. 307.
 Naval Staff vol. Xv. p. 139.
 Halpern, Naval, p. 307.
 Naval Staff vol. Xv. p. 139.
 Quoted in R. H. Gibson, M. Prendergast, The German Submarine War, 1914-1918. (London: Constable, 1931), p. 88.
 Ibid., p. 89.
 Halpern, Naval, p. 385.