In September and October 1916, the main task of the submarines UB-42 and UB-14 was patrolling the Russian and Romanian coasts from Constania to Sevastopol.  On September 30, 1916, near the port of Sulina, UB-42 launched a torpedo at the Romanian torpedo Smeul, but failed. The Romanian warship attacked, damaged the submarine`s periscope and tower and forced them to withdraw.    In November, the German submarine UC-15 was sent on a mine storage mission off Sulina and never returned as it was sunk by its own mines.   This was probably due to a meeting with Smeul, whose captain in November 1916 surprised a German submarine near Sulina, who would never have returned to his base at Varna. It could only be UC-15, whose systems most likely broke down after being forced to sink into shallow waters when they hit the Romanian torpedo.  In August 1914, a flotilla of nine submarines sailed from their base in Helgoland to attack Royal Navy warships in the North Sea in the first submarine warfare patrol in history.  Their objective was to sink the capital ships of the great British fleet and thus reduce the numerical superiority of the Grand Fleet over the German high seas fleet. The first sorting was not a success. Only one attack was carried out when U-15 fired a torpedo at HMS Monarch (which failed). Two of the 10 submarines were lost.
During World War I, United States Navy warships were deployed in both the Atlantic and the Mediterranean, with the main objective of fighting German submarines and escorting convoys. American participation began with an event known as the “Return of the Mayflower” when the first six destroyers arrived in Queenstown, Ireland, in May 1917.  Despite their long journey, the squadron commander answered the question of when they would be ready to patrol: “We are ready now.” For the most part, all available U.S. destroyers and a large portion of submarines were deployed in 1917-18, with bases such as Queenstown, Bantry Bay, the Azores and other locations. Numerous contacts and attacks have been established in the Atlantic and Mediterranean, although only two submarines have been sunk or hindered by American actions. A U.S. auxiliary cruiser severely damaged a submarine during the operation of April 4, 1918. The Germans then sailed directly to Spain, where they crushed their boat.
During the war, American submarine hunters also fought against Austro-Hungarian troops. Although their participation in the conflict was conceived as a counter-submarine effort, they were engaged by enemy coastal batteries, mapped a path through a minefield and helped sink two Austro-Hungarian destroyers at the Durazzo naval base in Albania. From April 1917, Japan, an ally of the United Kingdom, sent a total of 14 destroyers to the Mediterranean with cruiser flagship ships stationed in Malta and playing an important role in escorting convoys to protect them from enemy submarines.