U-boat technology was used to attack shipping by Germany during World War I. It threatened British civilians and war materiel production. U-boats were small submerged targets and difficult to identify and attack. They were effective until radar came along. But by the early stages of World War II, radar was a new technology and many U-boat captains chose not to use their U-boat radar systems.
The German Navy used U-boats as a means of hunting convoys, as well as combat vessels and civilian shipping. U-boats shadowed convoys and attacked at night, when they were unable to be escorted. Most U-boats were submerged by mines or depth charges. The U-boats were able to achieve great success in the Indian Ocean.
The German Navy designed larger U.boats to break the British blockade. U-boats could travel more quickly and had more power. They also had a longer submersion time than their smaller predecessors.
To attack British merchant shipping, the Germans initially used U-boats. A number of attacks occurred, including the sinking of the RMS Lusitania off Ireland. U-boats were opposed by the American government, which joined the war. It was concerned that U.boats could be used by the German Government to break diplomatic ties with the United States.
Allies were stunned at the U-boat attacks on British merchant ships. They were capable of destroying the merchant marine, which supplied vital supplies to Britain. The British blockade threatened Germany's exclusion from the war. U-boat supplies ships were attacked by the Allies. U-boat bases were also attacked with strategic bombardment. This method of antisubmarine war became the Battle of the Atlantic. The Allies' strategy was to attack U-boat supply vessels, while putting all of their shipping into convoys. The U-boats were unable to find targets, so this tactic proved less successful.
German U-boats started hunting in packs, also known as wolfpacks. They attacked convoys as well as military ships at night, when air cover was limited. They attacked both civilian and military shipping. The huge gap in mid Atlantic air cover made them very effective.
The German government responded by sending more U.boats to North Sea and directing the navy that it would follow the Prize Rules. This order was meant to restrain U-boat activities through the end of World War II. U-boats were returned to the coast waters close to the British Isles. They were also known as wolfpacks due to the close proximity of many U-boats.
Although the German government agreed to stop attacks on passenger liners, it continued to use U-boats against merchant shipping and civilian ships. By the end of the war, the U-boats had sunk more than 30% of the world's merchant ships. This was the highest rate of casualties of any German force during the war.