"S15" was a member of the 13th Half-flotilla of the 7th Torpedo boat flotilla of the German High Seas Fleet on the outbreak of war. The 7th Torpedo Boat Flotilla supported the Raid on Yarmouth on 3 November 1914 and the Raid on Scarborough, Hartlepool and Whitby on 16 December 1914. On the night of 19/20 December 1914, the two torpedo boats and of the 7th Flotilla stopped the Swedish steamer "Argo", suspected of carrying contraband to Russia near Simrishamn on the East coast of Sweden. "Argo" rammed "S22" and made off, ignoring a warning shot over her boxs, with the two torpedo boats not opening fire as the commanding officer was unsure whether they were in Swedish territorial waters. The Flotilla commander disagreed, arguing that "Argos"s hostile actions authorised the use of force, and ordered "S15" and to intercept "Argo" and to use "all means" to capture the ship. "Argo" was seized at Utlängan in Swedish waters, and taken to the German port of Swinemünde (now Świnoujście in Poland) but was released after a few hours. "S15", as part of the 7th flotilla, was part of the High Seas Fleet when it sailed to cover the Lowestoft Raid on 24–25 April 1916.