During World War I, Germany used U-boat technology to attack shipping, threatening the British population and production of war materiel. U-boats were small and submerged targets that were difficult to attack and detect. 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.
U-boats were used by Germany's Navy to hunt convoys, combat vessels, and civilian shipping. U-boats were used to attack convoys at night when they could not be escorted. Most U-boats were sink by mines and depth charges. However, U-boats still had a lot of success in Indian Ocean.
To break through the British blockade, the German Navy constructed larger U-boats. U-boats were faster than their predecessors and had more battery capacity. They could stay submerged for longer periods than their smaller predecessors.
Initially, the Germans used U-boats to target British merchant shipping. There were several attacks, including that of the RMS Lusitania (off Ireland). The U-boats were not allowed to be used by the American government when it joined the war. It was concerned that U.boats could be used by the German Government to break diplomatic ties with the United States.
The U-boat attacks on British merchant shipping were a surprise to the Allies. They were able disrupt the merchant shipping, which brought vital supplies to Britain. Germany could be forced to withdraw from the war by the British blockade. U-boat supply vessels were hit by the Allies' response. They also targeted U-boat bases with strategic bombing. This type of antisubmarine warfare is known as the Battle of the Atlantic. The Allies' strategy was attack U-boat supply vessels and put all their shipping in convoys. However, this strategy proved less effective because U-boats found it hard to locate targets.
German Uboats started hunting in groups, called wolfpacks. They attacked convoys and military ships at night, when the air cover was poor. They also destroyed civilian ships and passenger liners. The large gap in mid-Atlantic aerial cover was also a factor.
The German government reacted to the U boat attacks with more U boats in the North Sea. It also ordered that the navy follow Prize Rules. This order was aimed at restraining U-boat activities until the end of the war. U-boats were returned to the coast waters close to the British Isles. They were also called "wolfpacks" because they were made up several U-boats that were closely connected.
The German government had agreed to stop attacks against passenger liners but it continued to use Uboats against merchant shipping as well as civilian ships. The U-boats had already sunk over 30% of all world merchant ships by the end of World War II. This was the highest number of casualties suffered by any German force in the war.