U-20 was a Type U 19 submarine, launched on 18 December 1912 in Danzig and commissioned into the Imperial German Navy on 5 August 1913. She was a capable boat, but it was a single patrol in May 1915 that made her infamous. Three months before that fateful afternoon, Germany had declared the waters around the British Isles a war zone and any vessel within it a legitimate target. When Schwieger fired his torpedo from roughly 700 meters, it struck the Lusitania's starboard side almost directly below the bridge. He wrote in his war diary that the size of the explosion surprised him -- a second blast followed the torpedo's impact, its cause debated ever since. Coal dust, a boiler, illegal munitions: the theories have never been fully settled. What is certain is that Lusitania sank in just eighteen minutes, and that the Allies and the United States initially believed two torpedoes had been fired. Postwar investigations confirmed only one.

Before Schwieger could dock at Wilhelmshaven for refueling, the United States had lodged a formal protest in Berlin. Kaiser Wilhelm II, reading the American note, scrawled in its margins: "Utterly impertinent," "outrageous," and "this is the most insolent thing in tone and bearing that I have had to read since the Japanese note last August." The Kaiser's indignation was private; his policy reversal was not. To keep America out of the war, he rescinded unrestricted submarine warfare in June 1915, ordering that passenger liners be left alone. But Schwieger was not finished. On 4 September 1915, back at sea near the Fastnet Rock off southern Ireland, U-20 attacked and sank the RMS Hesperian, a ship carrying some 800 passengers and doubling as a hospital ship. Schwieger was reprimanded by the Admiralty but showed no remorse. The German government, unwilling to admit the attack, reported the ship had struck a mine.

U-20's war ended not in battle but in mechanical failure. On 4 November 1916, her engines gave out and she ran aground on the Danish coast south of the village of Vrist, just north of Thorsminde. Her crew tried to scuttle her the following day, detonating explosives that damaged the bow but failed to destroy the boat entirely. She remained on that beach, a rusting curiosity, for nearly a decade. In 1925 the Danish government finished the job with what witnesses described as a spectacular explosion. The conning tower survived -- salvaged and placed on the front lawn of the Strandingsmuseum St. George in Thorsminde, where it remains to this day, salt-weathered and defiant.

In 1984, the American adventure novelist Clive Cussler and his National Underwater and Marine Agency claimed to have located U-20's submerged remains a short distance offshore. The find added another layer to a story that has never stopped generating debate. Was the Lusitania armed? Was she carrying contraband munitions for the British war effort? Did Schwieger know what he was shooting at before he fired? The questions matter because the sinking did more than kill 1,197 people -- it shifted American public opinion decisively against Germany. Though the United States would not officially enter the war for another two years, the outrage over the Lusitania created a political current that made neutrality increasingly difficult to sustain. From a single torpedo fired off the coast of Ireland, to a rusting hull on a Danish beach, U-20 compressed the vast machinery of world war into a story small enough to hold in your hands.