On May 7th 1915, a German U-Boat sank the RMS Lusitania off the south west coast of Ireland. Although Lusitania was later found to have been carrying military supplies to Britain from America, the loss of some 1,200 civilians, some of them American, forced the US President Woodrow Wilson to send word to the Germans that the USA would not tolerate any more American deaths.
The Kaiser felt compelled to order his U-Boat fleet not to sink neutral shipping even if heading for the British Isles. However, as the Royal Navy’s blockade of German ports became more effective, the German High Command debated with the Kaiser over the merits of re-introducing unrestricted submarine warfare.
On 31st January 1917, the German High Command took the fateful decision to start attacking all shipping in the Atlantic and the North Sea. In February some 100 ships were sunk around the British coast and this increased to 147 in March.
With many ships being American, Congress gave President Wilson an ultimatum to declare war on Germany and America officially entered the Great War in April 1917.
With the advent of the convoy system under the protection of the British and American navies, the U-Boat threat was halted and the arrival of American troops in Europe began to swing the balance of the war against the Germans.
