During World War I, Germany used U-boat technology to attack shipping, threatening the British population and production of war materiel. U-boats had small, submerged targets which were difficult for attackers to find. They were very effective up until radar was invented. Radar was still a relatively new technology, and many U-boat Captains decided not to use the U-boat radar system in World War II.
The German Navy used U-boats as a means of hunting convoys, as well as combat vessels and civilian shipping. U-boats attacked at night and shadowed convoys when they couldn't be escorted. The U-boats fell mainly to depth charges and mines. However, the U-boats did achieve considerable success in the Indian Ocean.
To break through the British blockade, the German Navy constructed larger U-boats. The U-boats were able to travel much faster and had much greater battery capacity. They were also able to stay submerged longer than their smaller predecessors.
To strike British merchant shipping, U-boats were used initially by the Germans. There were several attacks, including that of the RMS Lusitania (off Ireland). U-boats were objected by the American government after it joined in 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 surprised by U-boat attacks against British merchant shipping. They were able sabotage the merchant navy, which was carrying vital supplies to Britain. The British blockade threatened to starve Germany out of the war. U-boat ships of supply were the first targets of the Allies' strikes. U-boat bases were also hit with strategic bombing. This method of antisubmarine war became the Battle of the Atlantic. Allies tried to destroy U-boat supply boats and then put all of their shipping into convoys. This tactic proved to be less effective due to U-boats finding targets more difficult with convoys.
German U-boats were known to hunt 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. They were also successful because of the large gap in midatlantic air coverage.
The German government responded to U-boat attacks by sending more Uboats to North Sea, and ordered the navy to adhere to Prize Rules. This order was aimed at restraining U-boat activities until the end of the war. U-boats returned from the war to coast waters near 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 worst German force to suffer casualties during World War II.