Kapitanleutnant Walther Schwieger commanded U-20 as it left Emden, Germany on 30 April 1915 heading northwest across the North Sea, planning to enter the Irish Sea and attack ships going in and out of Liverpool. During the 5/6 May, U-20 sank several merchant ships but Schwieger allowed their crews to evacuate before sinking them. The survivors of these sinkings were rescued, yet the Lusitania received no specific warnings as it entered the war zone.
On the morning of 7 May, visibility was poor and U-20 was low on fuel and had just three torpedoes. The British cruiser Juno, having received warning of submarine activity off Queenstown, passed the submerged submarine as it headed home. The Lusitania was sighted at 1:20 p.m. and Schwieger ordered the submarine to submerge. Only one torpedo was fired which caused two explosions, first from the initial strike and then an internal explosion (caused by illegal munitions according to German sources). Schwieger could see the unfolding chaos aboard the Lusitania and refused to “fire a second torpedo into this crushing crowd of humanity trying to save their lives.”
U-20 maintained radio silence on the attack until she was almost back to Germany, where the news was first received as a great triumph. The United States formally protested to Berlin against the sinking, and Germany then sought save her international reputation and prevent the United States entering the war. Kaiser Wilhelm officially suspended unrestricted submarine warfare.