Prior to World War I, a series of conferences were held at
Whitehall in 1905–1906 concerning military cooperation with France in the event of a war with Germany. The
Director of Naval Intelligence—
Charles Ottley—asserted that two of the Royal Navy′s functions in such a war would be the capture of German commercial shipping and the blockade of German ports. A blockade was considered useful for two reasons: it could force the enemy′s fleet to fight and it could also act as an economic weapon to destroy German commerce. It was not until 1908, however, that a blockade of Germany formally appeared in the Navy′s war plans and even then some officials were divided over how feasible it was. The plans remained in a state of constant change and revision until 1914, the Navy undecided over how best to operate such a blockade.
Meanwhile, Germany had made no plans to manage her wartime food supplies since in peacetime, she was able to produce some 80% of her total consumption. Furthermore, overland imports from the Netherlands, Scandinavia and Roumania would be unaffected by any naval blockade. However, the combined issues of conscription of farm labourers, the requisition of horses, poor weather, and the diversion of nitrogen from fertiliser manufacture into military explosives, all combined to cause a considerable drop in agricultural output.
The British—with their overwhelming sea power—established a naval blockade of Germany immediately on the outbreak of war in August 1914, issuing a comprehensive list of
contraband that all but prohibited American trade with the Central powers, and in early November 1914 declared the
North Sea to be a War Zone, with any ships entering the North Sea doing so at their own risk.
 The blockade was unusually restrictive in that even foodstuffs were considered "contraband of war". There were complaints about breaches of international law, however most neutral merchant vessels agreed to dock at British ports to be inspected and then escorted—less any "illegal" cargo destined for Germany—through the British minefields to their destinations.
Northern Patrol and
Dover Patrol closed off access to the
North Sea and the
English Channel respectively.
The Germans regarded this as a blatant attempt to starve the German people into submission and wanted to retaliate in kind.
The blockade also had a detrimental effect on the U.S. economy. Under pressure especially from commercial interests wishing to profit from wartime trade with both sides, the U.S. government protested vigorously. Britain did not wish to antagonize the U.S., but cutting off trade to the enemy seemed a more pressing goal. Eventually, Germany′s submarine campaign and the subsequent sinking of the
RMS Lusitania and other civilian vessels with Americans on board did far more to antagonize U.S. opinion than the blockade.
A memorandum to the British War Cabinet on 1 January 1917 stated that very few supplies were reaching Germany or its allies either via the North Sea or other areas such as Austria's
Adriatic ports (which had been subject to a French blockade since 1914).
Effects during the war
The first official accounts of the blockade—written by Professor A. C. Bell and
Sir James E. Edmonds, differed in their accounts of its effects upon German food supplies. Bell—who employed German data—argued that the blockade led to revolutionary uprisings in Germany and caused the collapse of the Kaiser′s administration.
 Edmonds, on the other hand, supported by Colonel Irwin L. Hunt (who was in charge of civil affairs in the American occupied zone of the Rhineland), held that food shortages were a post-armistice phenomenon caused solely by the disruptions of the
German Revolution of 1918–19.
More recent studies also disagree on the severity of the blockade′s impact on the affected populations at the time of the revolution and the armistice. Some hold
 that the blockade starved Germany and the Central Powers into defeat in 1918, but others maintain that while the German population did indeed go hungry as a result of the blockade, Germany′s rationing system kept all but a few from actually starving to death. German success against the Russians on the
Eastern Front culminating in the
Treaty of Brest-Litovsk gave Germany access to the resources of Poland and other eastern territories, which did much to counter the effects of the blockade. The armistice on 11 November was forced by events on the
Western Front rather than any actions of the civilian population. Also Germany's largest ally
Austria-Hungary had already
signed an armistice on 3 November 1918, exposing Germany to an invasion from the south.
Nevertheless, it is still accepted that the blockade made a large contribution to the outcome of the war; by 1915, Germany′s imports had already fallen by 55% from their prewar levels and the exports were 53% of what they were in 1914. Apart from leading to shortages in vital raw materials such as coal and non-ferrous metals, the blockade also deprived Germany of supplies of fertiliser that were vital to agriculture. This latter led to staples such as grain, potatoes, meat, and dairy products becoming so scarce by the end of 1916 that many people were obliged to instead consume
ersatz products including Kriegsbrot ("war bread") and powdered milk. The food shortages caused looting and riots, not only in Germany, but also in Vienna and Budapest.
 The food shortages got so bad that Austria-Hungary hijacked ships on the Danube that were meant to deliver food to Germany. Additionally, during the winter of 1916 to 1917, there was a failure of the potato crop, which resulted in the urban population having to subsist largely on
Swedish turnips; this period became known as the Steckrübenwinter or
The German government made strong attempts to counter the effects of the blockade; the
Hindenburg Programme of German economic mobilisation launched on 31 August 1916, was designed to raise productivity by the compulsory employment of all men between the ages of 17 and 60, and a complicated rationing system initially introduced in January 1915 aimed to ensure that a minimum nutritional need was met, with "war kitchens" providing cheap mass meals to impoverished civilians in larger cities. All these schemes enjoyed only limited success, and the average daily diet of 1,000 calories was insufficient to maintain a good standard of health, resulting by 1917 in widespread disorders caused by malnutrition such as
German official statistics estimated 763,000 civilian malnutrition and disease deaths were caused by the blockade of Germany.
 This figure was disputed by a subsequent academic study that put the death toll at 424,000.
 In December 1918 a German government report estimated that the blockade was responsible for the deaths of 762,796 civilians, the report claimed that this figure did not include deaths due to the
Spanish flu epidemic of 1918. The figures for the last six months of 1918 were estimated.
Maurice Parmelle maintained that "it is very far from accurate to attribute to the blockade all of the excess deaths above pre-war mortality", he believed that the German figures were "somewhat exaggerated".
 The German claims were made at the time when Germany was waging a propaganda campaign to end the Allied blockade of Germany after the armistice that lasted from November 1918 until June 1919. Also in 1919 Germany raised the issue of the Allied blockade to counter charges against the German use of submarine warfare.
In 1928 a German academic study sponsored by the
Carnegie Endowment for International Peace provided a thorough analysis of the German civilian deaths during the war. The study estimated 424,000 war related deaths of civilians over age 1 in Germany, not including
Alsace-Lorraine, the authors attributed these civilian deaths over the pre war level primarily to food and fuel shortages in 1917–1918. The study also estimated an additional 209,000
Spanish flu deaths in 1918
 A study sponsored by the
Carnegie Endowment for International Peace in 1940 estimated the German civilian death toll at over 600,000. Based on the above-mentioned German study of 1928 they maintained that "A thorough inquiry has led to the conclusion that the number of "civilian" deaths traceable to the war was 424,000, to which number must be added about 200,000 deaths caused by the influenza epidemic"
Blockade after armistice
The blockade was maintained for eight months after the Armistice in November 1918, into the following year of 1919. According to the
New Cambridge Modern History food imports into Germany were controlled by the Allies after the
Armistice with Germany until Germany signed the
Treaty of Versailles in June 1919.
 The total blockade was lifted on 17 January 1919 when the Allies allowed the importation of food under their supervision. The Allies requested that the German government send German merchant ships to Allied ports to transport food supplies. However the Germans considered the armistice a temporary cessation of the war and refused, believing that should fighting break out again the ships would be confiscated.
 The German government notified an American representative in Berlin that the shortage of food would not become critical until late spring. Food deliveries were delayed until March 1919 when the German government agreed to the restrictions imposed by the Allies. From March food imported from America in American ships arrived in Germany.
 The restrictions on food imports were finally lifted on 12 July 1919 after Germany signed the Treaty of Versailles.
C. Paul Vincent maintains that for the German people, these were the most devastating months of the blockade because "in the weeks and months following the armistice, Germany's deplorable state further deteriorated."
 However, Sally Marks believes that the German accounts of a hunger blockade are a "myth". She points out that although the Germans had denied Belgium and northern France food during the war, leading to starvation, the Allies made no effort to deny Germany food. According to Marks the food situation in 1919 in Belgium, northern France and Poland was worse because the Germans had confiscated the harvest.
Not included in the German government's December 1918 figure of 763,000 deaths are civilian famine related deaths during 1919. A recent academic study maintains that there is no statistical data for the death toll of the period immediately following the Armistice in November 1918.
Max Rubner in an April 1919 article claimed that 100,000 German civilians had died due to the continuation blockade of Germany after the armistice.
 In the UK a
Labour Party anti-war activist
Robert Smillie issued a statement in June 1919 condemning continuation of the blockade, and claiming that 100,000 German civilians had died.