The U-boat campaign from 1914 to 1918 was the World War I naval campaign fought by German U-boats against the trade routes of the Allies, largely in the seas around the British Isles and in the Mediterranean, as part of a mutual blockade between the German Empire and the United Kingdom.
Both Germany and Britain relied on food and fertilizer imports to feed their populations, and raw materials to supply their war industry. The British Royal Navy was superior in numbers and could operate on most of the world's oceans because of the British Empire, whereas the Imperial German Navy surface fleet was mainly restricted to the German Bight, and used commerce raiders and submarine warfare to operate elsewhere.
German U-boats sank almost 5,000 ships with over 12 million gross register tonnage, losing 178 boats and about 5,000 men in combat. The Allies were able to keep a fairly constant tonnage of shipping available, due to a combination of ship construction and countermeasures, particularly the introduction of convoys.
In August 1914, a flotilla of ten U-boats sailed from their base in Heligoland to attack Royal Navy warships in the North Sea in the first submarine war patrol in history. Their aim was to sink capital ships of the British Grand Fleet, and so to reduce the Grand Fleet's numerical superiority over the German High Seas Fleet. The first sortie was not a success. One of SM U-9's engines broke down and she had to return to Heligoland. Only one attack was carried out, when U-15 fired a torpedo (which missed) at HMS Monarch. Two of the ten U-boats were lost.
Later in the month, the U-boats achieved success, when U-21 sank the cruiser HMS Pathfinder. In September, SM U-9 sank three armoured cruisers (Aboukir, Hogue, and Cressy) in a single action. Other successes followed. In October U-9 sank the cruiser Hawke and U-27 sank the submarine E3, the first time one submarine sank another, and on the last day of the year SM U-24 sank the pre-dreadnought battleship Formidable. By the end of the initial campaign, the U-boats had sunk nine warships while losing five of their own number.
Due to fears of submarine attack, the Grand Fleet spent much of the first year dispersed on the West coast, while their home base at Scapa Flow had defenses installed.
The initial phase of the U-boat campaign in the Mediterranean comprised the actions by the Austro-Hungarian Navy's U-boat force against the French, who were blockading the Straits of Otranto. At the start of hostilities, the Austro-Hungarian Navy had seven U-boats in commission; five operational, two training; all were of the coastal type, with limited range and endurance, suitable for operation in the Adriatic. However during the war new larger U-boats came into service plus Germany shipped several overland. The Austro-Hungarian U-boats had a number of successes. On 21 December 1914 U-12 torpedoed the French battleship Jean Bart (beyond Otranto), causing her to retire to Malta for serious repairs, and on 27 April 1915 U-5 sank the French cruiser Léon Gambetta, with a heavy loss of life.
While U-boats could sink a large and expensive armored warship with one torpedo, they needed to be in position before an attack took place. Their fastest speed, while surfaced, of around 15 knots, was less than the cruising speed of most warships and only two-thirds that of the most modern dreadnoughts. Their chief advantage was to submerge, because surface ships had no means to detect a submarine underwater, and no means to attack. However, while submerged, the U-boat was virtually blind and immobile, as early submarines had limited underwater speed and endurance.
The U-boats scored a number of impressive successes early in the war, but warships adopted tactics to counter them. Whilst warships were traveling at speed and on an erratic zigzag course they were relatively safe, and for the remainder of the war, the U-boats were unable to mount a successful attack on a warship traveling in this manner.
Overall, the German Navy was unable to erode the Grand Fleet's advantage as hoped. In the two main surface actions of this period, the U-boat was unable to have any effect, and the High Seas Fleet was unable to draw the Grand Fleet into a U-boat trap.
First attacks on merchant ships
The first attacks on merchant ships had started in October 1914. At that time there was no plan for a concerted U-boat offensive against Allied trade. It was recognised that the U-boat had several drawbacks as a commerce raider, and such a campaign risked alienating neutral opinion. In the six months to the opening of the commerce war in February 1915, U-boats had sunk 19 ships, totalling 43,000 GRT.
Unrestricted submarine warfare
Because Germany could not possibly deal with British naval strength on an even basis, the German navy was relatively inactive at the start of the war, yet eager to demonstrate a role for itself. Throughout 1914, figures like Hermann Bauer, Alfred von Tirpitz and Hugo von Pohl argued that submarine commerce raiding provided a means of quickly defeating Britain. Perhaps influenced by the appearance of submarines in fiction, naval officials proposed extremely optimistic views of how effective even a very a small U-boat (with as few as 4–20 vessels) blockade could be. Such views were readily taken up by the German public, as by early 1915, all the combatants had lost the illusion that the war could be won quickly, and began to consider harsher measures in order to gain an advantage.
The British, with their overwhelming sea power, had established a naval blockade of Germany immediately on the outbreak of war in August 1914, and in early November 1914 declared the North Sea to be a "Military Area". Any ships entering were advised to pass through specific lanes (inspected by the British) or risk striking a growing array of minefields. While the word "blockade" was avoided in official pronouncements, this amounted to unprecedented restrictions on trade with the Central Powers, with even food considered "absolute contraband of war". Though at this point Germany was still receiving sufficient imports from neutral countries, Germans regarded this as a blatant attempt to starve the German people into submission and wanted to retaliate in kind, and in fact the severity of the British blockade did not go over well in America either. This gave the Germans the pretext to act. The German Chancellor, Theobald von Bethmann Hollweg, felt that such a submarine blockade, based on "shoot without warning", would simply antagonise the United States and other neutrals and have little chance of achieving its objectives. However, he was unable to hold back the pressures for taking such a step. The Chancellor and the Admiralty came to an agreement on 1 February and directives were sent out the next day.
On 4 February 1915 Admiral Hugo von Pohl, commander of the German High Seas Fleet and Head of the Admiralty Staff until 1 Feb, published a warning in the Deutscher Reichsanzeiger (Imperial German Gazette):
(1) The waters around Great Britain and Ireland, including the whole of the English Channel, are hereby declared to be a War Zone. From February 18 onwards every enemy merchant vessel encountered in this zone will be destroyed, nor will it always be possible to avert the danger thereby threatened to the crew and passengers.
(2) Neutral vessels also will run a risk in the War Zone, because in view of the hazards of sea warfare and the British authorization of January 31 of the misuse of neutral flags, it may not always be possible to prevent attacks on enemy ships from harming neutral ships.