For the very same reason US subs were a danger to Japan during WWII. Japan & Britain are both Islands. Islanders depend upon IMPORTS. Subs sink ships. Ships IMPORT!
U-boats
German U-boats were used to sink mainly merchant ships, so as to starve Britain of food supplies and ammunition.
Great Britain & Germany & the US east coast.
nationalism
they were rationed because the foods were being brought into Britain by boat and the boats were being sunk by German U-boats.
no
German U-boats attacked Allied shipping convoys in the North Atlantic to try and starve Britain into submission and they nearly succeeded.
If you are meaning to U-boats as in World War two? Than U-boats certainly mean "Underwater Boats"
they used U-boats, which are German submarines.
They were supplying Germany's enemy...Great Britain.
Because they were supplying Germanys enemy; Britain.
U-boats were for attacking enemy shipping.