Explanation:

World War I had begun in 1914. On the western front, the British and the French were fighting against the advancing Germans. New supplies for the war effort were transported using shipping lanes around Britain. At first, the Germans tried to gain control of the shipping lanes using their navy, but the British managed to keep the German navy in check.
The situation in the waters around Britain changed as the Germans began to use submarines to attack ships. They called their submarines "Unterseeboots" or "undersea boats". This name was shortened to U-boats. On February 4, 1915, the Germans declared the seas around Britain a war zone and said they would attack any Allied ship that entered the region.