 From the early stages of the war the British had blockaded Germany , preventing neutral shipping from reaching German ports . By the time of the so @-@ called " turnip winter " of 1916 – 17 , the blockade had severely limited imports of food and fuel into Germany . Among the results were an increase in infant mortality and as many as 700 @,@ 000 deaths attributed to starvation or hypothermia during the war . With the blockade having such dire consequences , Kaiser Wilhelm II personally approved a resumption of unrestricted submarine warfare to begin on 1 February 1917 to help force the British to make peace . The new rules of engagement specified that no ship was to be left afloat . 
