Military submarines first made a significant impact in World War I. Forces such as the U-boats of Germany saw action in the First Battle of the Atlantic, and were responsible for the sinking of Lusitania, which was sunk as a result of unrestricted submarine warfare and is often cited among the reasons for the entry of the United States into the war.
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 reduce the Grand Fleet's numerical superiority over the German High Seas Fleet. With much depending more on luck than strategy, the first sortie was not a success. Only one attack was carried out, when U-15 fired a torpedo (which missed) at HMS Monarch, while two of the ten U-boats were lost. The U-9 had better luck. On 22 September 1914 while patrolling the Broad Fourteens, a region of the southern North Sea, U-9 found a squadron of three obsolescent British Cressy-class armoured cruisers (HMS Aboukir, HMS Hogue, and HMS Cressy), which were assigned to prevent German surface vessels from entering the eastern end of the English Channel. She fired all six of her torpedoes, reloading while submerged, and sank all three in less than an hour.
The U-boats' ability to function as practical war machines relied on new tactics, their numbers, and submarine technologies such as combination diesel-electric power system developed in the preceding years. More submersibles than true submarines, U-boats operated primarily on the surface using regular engines, submerging occasionally to attack under battery power. They were roughly triangular in cross-section, with a distinct keel to control rolling while surfaced, and a distinct bow. During World War I more than 5,000 Allied ships were sunk by U-boats.Submarine in World War I:
| Submarine | Picture | Specifications | Country | Year |
|---|---|---|---|---|
| British R class submarine | UK | 1918 | ||
| H | US | 1918 | ||
| L | US | 1918 | ||
| M1 | US | 1918 | ||
| N | US | 1918 | ||
| O | US | 1918 | ||
| R | US | 1918 | ||
| British L class submarine | UK | 1917 | ||
| British K class submarine | UK | 1917 | ||
| American Holland class submarine | Russia | 1916 | ||
| British J class submarine | UK | 1916 | ||
| Bars class submarine | Russia | 1915 | ||
| British G class submarine | UK | 1915 | ||
| British H class submarine | UK | 1915 | ||
| CC class submarine | Canada | 1914 | ||
| Norwegian A class submarine | Norway | 1914 | ||
| Morzh class submarine | Russia | 1914 | ||
| Russian Narval class submarine | Russia | 1914 | ||
| Nautilus class submarine | UK | 1914 | ||
| G | US | 1914 | ||
| British F class submarine | UK | 1913 | ||
| F | US | 1913 | ||
| British E class submarine | UK | 1912 | ||
| E | US | 1912 | ||
| K | US | 1912 | ||
| Kaiman class submarine | Russia | 1910 | ||
| C | US | 1910 | ||
| D | US | 1910 | ||
| British D class submarine | UK | 1909 | ||
| Russian submarine Pochtovy | Russia | 1908 | ||
| Russian submarine Minoga | Russia | 1908 | ||
| Russian submarine Akula | Russia | 1908 | ||
| Karp class submarine | Russia | 1907 | ||
| Karp class submarine | Russia | 1907 | ||
| B | US | 1907 | ||
| British C class submarine | UK | 1906 | ||
| Som class submarine | Russia | 1905 | ||
| Osetr class submarine | Russia | 1905 | ||
| British B class submarine | UK | 1905 | ||
| Kasatka class submarine | Russia | 1904 | ||
| Russian submarine Forel | Russia | 1904 | ||
| Russian submarine Delfin | Russia | 1903 | ||
| Holland class submarine | UK | 1903 | ||
| Plunger | US | 1903 | ||
| Holland SS1 | US | 1900 | ||
| Gymnote (Q1) | France | 1888 | ||
| Alligator | US | 1862 |
