On 1 February 1917, Germany declared unrestricted submarine warfare; with its U-boats torpedoing ships without warning, allowing passengers and crew little or no chance of escape.
Nicknamed Der Magische Guertel, or the Magic Girdle, the operation aimed to starve Britain into submission.
All ships trading with Britain were seen as targets, including those from neutral countries such as Norway, Denmark and the United States. The targeting of American ships ultimately brought the United States into the war in April 1917.
Even hospital ships were targeted by German forces during unrestricted submarine warfare, provoking worldwide outrage. Ships like HMHS Rewa, torpedoed in January 1918, were unarmed and painted with the internationally recognised symbol of the Red Cross but were still sunk.
Wrecked on the coast
To mark the anniversary of this turning point in the war, Historic England has published a selection of images that depict U-boats that were wrecked on the Cornish coast following in the end of the First World War in 1918.
The submarines had been surrendered by Germany and were on their way to be used as gunnery targets by the British Royal Navy.
They had been stripped of their engines and, buoyant and hard to tow, some sank or were wrecked on the British coast.
The photographs were taken in 1921 by naval officer Jack Casement (pictured above).
Also marking this year’s anniversary, the National Submariners’ memorial at Temple Pier on the Thames has also been upgraded to a Grade II* listing.
Unveiled in 1922, it commemorates the one third of the Submarine Service’s total personnel who lost their lives during the First World War – the highest proportion of any branch of the armed services.
Roger Bowdler, director of Listing at Historic England, said: ‘The declaration of unrestricted submarine warfare in February 1917 was a decisive moment in the First World War. Germany’s tactic led to devastating losses for many nations but it also horrified the world.
‘It was seen as uncivilised, ungentlemanly and ultimately brought the might of the United States into the war. By commemorating this day we can better understand its consequences and remember the many people who lost their lives in this way.’