During the First World War, Britain intended to use its powerful navy to starve Germany and Austria-Hungary into submission. By maintaining a blockade of enemy ports it hoped to cut off supplies from the outside world. The consequences of this strategy were complex.
The Royal Navy followed a policy of 'distant blockade', barring entrance to the English Channel and the North Sea. A similar blockade was maintained in the Adriatic Sea, with French and Italian aid. Neutral vessels were theoretically permitted to continue trading, but Britain progressively widened the definition of 'contraband' cargo and, from early 1915 began to seize all commodities bound for the Central Powers. This policy made for difficult relations with neutral countries, particularly the United States.
Germany and Austria-Hungary managed to develop substitutes for many materials which were essential for their war effort. They were less successful in feeding their citizens – despite the fact that they had not relied upon imported food before the war. Central Powers propaganda blamed food shortages on the British 'Hunger Blockade', but a combination of bad harvests and incompetent regulation of food distribution made the situation far worse.
Germany's civilians began to suffer malnourishment from the winter of 1916 onwards, while the food situation in Austria caused riots and, eventually, actual starvation in some areas. A wish to retaliate and to break Britain’s command of the seas motivated Germany to launch its campaign of unrestricted submarine warfare in 1917. The result was to make the blockade even more complete, by provoking the United States to join the Allies.