The U-boat was used during World War I and helped the Germans destroy many Allied ships. These naval battles mostly took place around the British Isles and in the Mediterranean Sea. The primary targets were trade routes.
Both the United Kingdom and Germany relied on heavy imports for their war efforts and to feed their populations, which is why Germany and the UK did their best to blockade each other. However, as the British Royal Navy fleet had a superior numbers of ships, it allowed the British Empire to operate freely across much of the world. In an effort to stop them, the Germans deployed their U-boats.
Throughout World War I, German U-boats managed to sink nearly 5,000 ships, while losing only 5,000 men and a total of 178 U-boats. Overall, 375 U-boats were commissioned by the Imperial German Navy before the end of war. The U-boat's first success was during August of 1914 when the U-boat SM U-21 sank the British light cruiser, HMS Pathfinder. On September 22 of the same year, SM U-9 managed to sink three British armored cruisers, HMS Aboukir, HMS Hogue and HMS Cressy within an hour, each weighing 12,000 tons.
After the end of the war, the Allies were so impressed with the German submarine engineering of the U-boats that each of the Allied countries took a share of the ships. They used them to engineer their own versions, including Britain's X1 submarine, which was launched in 1923.
U-boats relied on torpedoes as weaponry and were typically capable of traveling fully submerged for two hours. The Treaty of Versailles, which marked the end of World War I, prohibited Germany's possession of U-boats but did not prevent the country from researching U-boat technology, resulting in the more advanced U-boats that were used by Germany in World War II.