An act of villainy
In August 1915, the British E-class submarine, HMS E13, attempted to navigate the perilously narrow and shallow Kattegat between Denmark and Sweden to penetrate into the Baltic. There she was to reinforce the Royal Navy's Baltic submarine flotilla, sent there to assist the Russians in prevention of German iron ore imports from Sweden.
Sadly, due to an error in her compass, E13 ran aground on the island of Saltholm, within Danish territorial waters. The Danish Navy offered the submarine's captain an ultimatum: refloat the boat and leave Danish territorial waters within 24 hours or the crew would be interned for the rest of the war. The officers and men worked desperately to offload stores and ammunition to lighten the submarine, but all the activity attracted the interest of the Imperial German Navy. Angry at the mayhem the Baltic submarine flotilla was already creating, the local German admiral ordered a German destroyer and torpedo boat to destroy the submarine, despite its presence in Danish waters.
The German ships first torpedoed the submarine and then opened fire on her, quickly causing her to catch fire. Swimmers in the water were machine gunned until the Danish Navy intervened. 15 British sailors were killed.
This account is just one that I relate in my novel, Where the Baltic Ice is Thin, about the successful, but forgotten campaign by Royal Navy submarines in the Baltic during WW1.