During the First World War both the United Kingdom and Germany attempted to carry out naval blockades of the other. Each country used actions by the other to justify escalations by the other. The Germans laid unanchored, floating mines in the North Sea early in the war. International law said that such mines had to become inoperable after an hour. It was not feasible to do so, but this gave the UK an opportunity to argue that it was Germany that had first broken international law.
The UK then tightened its blockade beyond what was permitted by the 1909 Declaration of London. On 5 November the Admiralty announced that the whole of the North Sea was a war zone, warning that ships entering from the north would do so at their own risk. The official reason for this was the German minefields, but the real one was to force neutral ships heading into the North Sea to go through the English Channel, making it easier to search them for contraband.
On 4 February 1915 Germany announced that the waters round the United Kingdom of Great Britain and Ireland would be a war zone from 18 February. It justified this by reference to the British declaration of 5 November. All hostile merchant ships in this area would be destroyed without warning. The safety of neutral ships could not be guaranteed because British ships had been flying neutral colours. U-boat captains were ordered that their first duty was the safety of their boat, so they should not take risks to find out if a ship was really neutral.
In 1914 the most important successes of German U-boats had been against warships. This continued in 1915, with U24 sinking the pre-dreadnought battleship HMS Formidable in the early hours of the year.
It was extremely difficult for submarines to comply with the rules of cruiser warfare, which required raiders to stop and search enemy merchant ships. Merchant ships could be sunk or captured only if their cargoes contained either war materials (absolute contraband) or items such as food or fuel that had peaceful uses but were intended for the enemy’s military (conditional contraband).
The safety of the crews of merchant ships that were sunk had to be ensured. Submarines had no space for prisoners and no spare men to act as prize crews for captured ships, so could comply with this only if the sinking occurred close to land.
One man had predicted before the war that submarines would sink merchant ships without warning. This was the British Admiral Lord Fisher, who by February 1915 was in his second spell as First Sea Lord. In late 1913 he had written a paper arguing that the invention of the submarine meant that the main threat to the UK was having its food and oil supplies cut off by submarine attacks on its merchant shipping
Winston Churchill, the First Lord of the Admiralty, and Admiral Prince Louis Battenberg, then the First Sea Lord, both refused to believe that any country would use its submarines in such a way. They thought that Fisher’s view that it would be done weakened his arguments about the risk of submarine attack on the UK’s food and oil supplies.
Only four British merchant vessels had been sunk by U-boats before 30 January. In each case the Germans had given the crew time to abandon ship and no lives were lost.
The most controversial incident had come on 26 October when Kapitänleutnant Rudolf Schneider’s U24 torpedoed but did not sink the 4,590 ton French steamer Amiral Ganteaume. She was carrying 2,500 Belgian refugees, 40 of whom were killed. However, Schneider may have thought that she was a troop ship. Whether he did or not, the incident showed the potential political and diplomatic implications of submarine warfare against merchant shipping.
In the Irish Sea on 30 January U21, which had sunk the cruiser HMS Pathfinder in September, the first moving warship ever to be sunk by a submarine, stopped, searched and sank three British merchantmen after allowing their crews time to take to their boats. On the same day, however, U20 torpedoed and sank three British merchant ships off Le Havre without warning. The crews of the Tokomaru and Ikaria all survived, but all that was found of the 21 man crew of the Oriole was two lifebuoys discovered near Rye on 6 February.
Before the war Kapitänleutnant Ulrich Blum of the Kaiserliche Marine, the German navy, had estimated that 222 U-boats would be needed to successfully carry out a war against British commerce under international law, far more than Germany possessed in early 1915.
The KM began the war with 24 operational U-boats, plus four older ones that were used for training and 16 under construction. By 22 February 13 more had been completed, but one of the new boats and six of the pre-war ones had been lost. Seven newly completed boats were under sea trials, leaving the KM theoretically with 23 to attack British trade. The need to refit, repair and resupply meant that there was an average of 5.6 and a maximum of 12 boats at sea on any one day between March and May 1915.
After the German capture of the Belgian coast gave them bases much closer to the UK, they had begun to build smaller UB and UC coastal submarines. The latter were minelayers, which initially were armed only with mines, although later ones also had torpedo tubes and guns. They were supposed to take four months to build compared with 18 for an ocean going boat, but UB1 was constructed in 75 days. By mid July 1915 17 UBs and 15 UCs had been completed, but the first six UBs were not operational until the latter part of April and UC1 not until early June. 
The U-boat war against British trade would eventually cause great problems for the UK, but Germany had too few submarines when it first launched its offensive. However, British counter-measures were also inadequate at first.
The Germans had lost a high proportion of their submarines so far in the war, but two had been rammed by warships whilst on the surface, four had struck mines and the seventh had been sunk by another U-boat on the surface after failing to respond to a challenge. The method that British warships possessed of sinking submerged submarines was the towing of explosive sweeps.
The British had laid a minefield across the Dover Straits in an attempt to stop U-boats entering the English Channel, but 4,000 out of just over 7,000 mines laid between 2 October 1914 and 16 February 1915 drifted or sank to the bottom because the weights holding them in position were too light. The British also laid nets and organised patrols of small, armed ships, but the U-boats were able to pass through the Dover Straits.
The orders issued to U-boat captains on 18 February were that they should attack all hostile ships except hospital ships, unless clearly acting as troopships, and Belgian Relief ships. Neutrals should be spared. However, they were told that ‘if, in spite of the exercise of great care , mistakes should be made, the commanding officers will not be held responsible.’
However, the first ship to be attacked in the new campaign was the Norwegian Belridge, which was carrying a cargo of oil from New Orleans to Amsterdam for the Dutch government. She was torpedoed on 19 February, but managed to make a British port. The Germans apologised and paid compensation.
 P. G. Halpern, A Naval History of World War I (London: UCL Press, 1994), pp. 293-94.
 R. H. Gibson, M. Prendergast, The German Submarine War, 1914-1918. (London: Constable, 1931), p. 24.
 C. E. Fayle, Seaborne Trade., 3 vols. (London: HMSO, 1920). vol. i, pp.358, 365-66.
 Halpern, Naval, p. 292.
 Fayle, Seaborne. vol. i, pp. 366-68.
 Halpern, Naval, p. 291.
 V. E. Tarrant, The U-Boat Offensive, 1914-1945 (London: Arms and Armour, 1989), p. 7.
 Ibid., p. 17.
 Gibson, Prendergast, German, p. 31.
 Fayle, Seaborne. vol. ii, p. 12