Submarine warfare was conducted by the Germans using U-boats to destroy merchant navy ships.

U-boats were German submarines which were used to sink enemy ships. In 1915, Germany had 21 U-boats and sank 4% of ships supplying Britain, despite the target being to destroy all merchant shipping. By 1917, the number of German U-boats had increased to 200, and they sank 841,114 tonnes of Allied shipping that year.

The Germans sank all ships entering British waters, regardless of which country they belonged to, until the sinking of the Lusitania in 1915 almost brought the USA into the war. This was known as unrestricted U-boat warfare. The policy was abandoned until 1917 when an increasingly desperate Germany tried it again, leading the USA to declare war on Germany.

U-boats were used to destroy merchant ships bringing in food and war materials from abroad. This was in retaliation for the naval blockade of German ports by Britain.

Overall, Germany's U-boat campaign did not have the desired effect:

• ❖ Although reduced to six weeks' food supply by 1917, the people of Britain did not starve due to rationing and anti U-boat measures.
• ❖ Likewise, the sinking of American ships brought the USA into the war in April 1917.