not to target passenger liners. In 1916 the United States launched a protest over a cross-channel passenger ferry sinking, Germany modified its rules of engagement. 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. The accompanying destroyers might sink a submerged submarine with depth charges. The losses to submarine attacks were reduced significantly. But the convoy system slowed the flow of supplies. The solution to the delays was a massive program to build new freighters. Various troop ships were too fast for the submarines and did not have to travel the North Atlantic in convoys. The First World War also saw the first use of aircraft carriers in combat, with launching Sopwith Camels in a successful raid against the Zeppelin hangars at Tondern in July 1918. Maritime events of World War II Battle of the Atlantic In the North Atlantic, German U-boats attempted to cut supply lines to the United Kingdom by sinking merchant ships. In the first four months of the war they sank more than 110 vessels. In addition to supply ships, the U-boats occasionally attacked British and Canadian warships. One U-boat sank the British carrier , while another managed to sink the battleship in her home anchorage of Scapa Flow. In the summer of 1941, the Soviet Union entered the war on the side of the Allies. Although the Soviets had tremendous reserves in manpower, they had lost much of their equipment and manufacturing base in the first few weeks following the German invasion. The Western Allies attempted to remedy this by sending Arctic convoys, which travelled from the United Kingdom and the United States to the northern ports of the Soviet Union: Archangel and Murmansk. The treacherous route around the North Cape of Norway was the site of many battles as the Germans continually tried to disrupt the convoys using U-boats, bombers, and surface ships. Following the entry of the United States into the war in December 1941, U-boats sank shipping along the East Coast of the United States and Canada, the waters around Newfoundland, the Caribbean Sea, and the Gulf of Mexico. They were