Conquering the Seas
German U-boats cut supply lines between North America and Britain. The nature of submarine warfare meant that attacks often came without warning, giving the crews of the merchant ships little hope of survival.The United States launched a protest, and Germany modified its rules of engagement. After the notorious sinking of the passenger ship RMS Lusitania in 1915, Germany promised not to target passenger liners, while Britain armed its merchant ships, placing them beyond the protection of the "cruiser rules" which demanded warning and placing crews in "a place of safety" (a standard which lifeboats did not meet). Finally, in early 1917 Germany adopted a policy of unrestricted submarine warfare, realizing the Americans would eventually enter the war. Germany sought to strangle Allied sea lanes before the U.S. could transport a large army overseas.
The U-boat threat lessened in 1917, when merchant ships entered convoys escorted by destroyers. This tactic made it difficult for U-boats to find targets, which significantly lessened losses; after the introduction of hydrophone and depth charges, accompanying destroyers might actually attack a submerged submarine with some hope of success.
The U-boats had sunk almost 5,000 Allied ships, at a cost of 178 submarines.World War I also saw the first use of aircraft carriers in combat, with HMS Furious launching Sopwith Camels in a successful raid against the Zeppelin hangars at Tondern in July 1918, as well as blimps for antisubmarine patrol.