The U-20 was a German Type U-19 U-boat in service in the 1910s. During World War I, it was under the command of Kapitänleutnant Walther Schwieger. On 7 May 1915, the U-20 torpedoed the RMS Lusitania, which despite being a neutral vessel was still in the blockade area and therefore a legitimate target. The sinking, and deaths of American neutrals, was a factor in the entry of the United States of America into the First World War.
In an alternative timeline, the Lusitania did not sink due to the Fifth Doctor's intervention. Eric Charles Vincent (who was killed in the sinking in the proper timeline) went on to kill Alexander Fleming in a botched robbery attempt in December 1927 before the biologist discovered penicillin. Without the protection offered by penicillin, Earth fell prey to new strains of meningitis and pneumonia in 1956. The survivors never developed a space programme. Consequently, the Knights of Velyshaa were not defeated by the Earth Alliance in 3562 as they otherwise would have been. (AUDIO: The Sirens of Time)