SMS S138 (1906)

In August 1914, after the outbreak of the First World War, S138 was listed as being part of the 3rd half-flotilla of the 2nd Torpedo Boat Flotilla, part of the High Seas Fleet.[12][13] From 3 to 9 September 1914, the German Baltic Fleet, supported by units of the Highs Seas Fleet, carried out a sortie into the Baltic with the intention of luring out the Russian Baltic Fleet out to battle. The 2nd Torpedo boat Flotilla, including S138, took part in this operation,[14][15][16] and on 6 September, S138, together with S139, S140, S141, S142 and 149, shelled a lighthouse and radio station on the island of Bogskär (off the coast of Finland), destroying the lighthouse, and capturing the station's crew of five men.[15][17] On 21 November 1914, S138 spotted a surfaced submarine just south of the Amrun Bank.[18]

In April 1915, the Baltic torpedo forces were reorganised and strengthened, with old torpedo boats from the 2nd Flotilla transferred as they were replaced by new ships. S138 joined the 20th half-flotilla, part of the newly established 10th Torpedo Boat Flotilla, which operated in the Baltic.[19][20] From 13 to 15 April 1915, S138, together with the torpedo boats G132 and S141, and the cruisers Prinz Adalbert, Lübeck and Thetis, escorted the minelayer Deutschland on a minelaying mission in the Gulf of Finland.[19][21] On 27 April 1915, the German Army launched an offensive in the Baltic to tie down Russian forces in advance of the start of the Gorlice–Tarnów offensive. On 7 May, German cruisers bombarded Russian positions around Libau (now Liepāja). S138 and the torpedo boat S141 carried out a reconnaissance of the entrances to Libau harbour, with the rest of the 10th Torpedo Boat Flotilla in support, while the torpedo boat S128 was mined and damaged during the operation. The Russians left Libau on the night of 7/8 May.[22][19][23] On 25 May, S138 and S142, which had just escorted the cruiser Augsburg into Libau after a minelaying mission, were sent out to search for a missing aircraft, which was later found by a patrol boat.[24]

From 3 June 1915, the Germans launched a major minelaying operation, in which their ships would pass through the Irbe Strait and mine the south entrance to the Moon Sound. The German force was split into several groups, with S138, S141, S142 and S149 escorting the cruisers Prinz Adalbert, Prinz Heinrich and Roon. The operation encountered heavy Russian opposition, and was abandoned, with the collier Dora Hugo Stinnes sunk and the torpedo boat S148 damaged by torpedoes from the British submarine E9 and the seaplane carrier Glyndwr damaged by a mine.[25][26][27] On the night of 18/19 June, S138 led five torpedo boats of the 19th half-flotilla (S126, S130, S131, S147 and S149) into the Moon Sound to lay mines.[28][27] On 2 July 1915, a German minelaying mission was intercepted by Russian cruisers in the Battle of Åland Islands. The armoured cruisers Prinz Adalbert and Prinz Heinrich, escorted by S139 and S138, set out in response, but E9 encountered the two cruisers and torpedoed Prinz Adalbert. The cruiser was damaged but remained afloat, and was escorted to safety by S139, while S138 attempted to attack the submarine, firing at E9's periscope, and attempting to ram.[27][29][30] In August 1915 the German Baltic Fleet, supported by a large portion of the High Seas Fleet, launched a major operation against Russian naval forces in the Gulf of Riga in support of the advance of German troops. It was planned to enter the Gulf via the Irben Strait, defeating any Russian naval forces and mining the entrance to Moon Sound.[31] On 8 August, a Russian flying boat that had just attacked Roon force landed nearby and S138 captured the aircraft's crew, preventing them from destroying the flying boat.[27][32]

By May 1916, S138 had transferred to the Vorpstenboot (outpost or patrol boat) flotilla protecting the mouth of the River Ems, serving as leader to the unit's 1st half-flotilla of nine patrol boats.[33] On 24 September 1917, S138 was renamed T138,[8] to free her old name for a new S138, a 1916 Mobilisation Type torpedo boat.[34][35] In early 1918, owing to the need to escort German U-boats through minefields in the German Bight, the coastal patrol and minesweeping forces of the German forces were reorganised to form large escort flotillas.[36] T136 joined one of these escort flotillas,[8] being part of the 3rd escort half-flotilla of the 1st Escort Flotilla in April 1918.[37]

On 6 July 1918, the 2nd escort half-flotilla, including T138, set out to escort the submarine U-55 through the minefields. In the early morning of 7 July 1918, the escort was about to turn back when a mine detonated in the wake of T148. Shortly afterwards, T138 struck a mine and broke in two, sinking within a few minutes, killing 32 of her crew.[38][8] The torpedo boat T172, which was following T138, also struck a mine and after the survivors of her crew were rescued, was scuttled.[38]