The Blockade of Germany, or the Blockade of Europe, occurred from 1914 to 1919. The prolonged naval blockade was conducted by the Entente during and after World War IThe New Cambridge Modern History, Vol 12 (2nd ed), Cambridge University Press, 1968, pp. 213 in an effort to restrict the maritime supply of goods to the Central Powers, which included Germany, Austria-Hungary, and the Ottoman Empire. The blockade greatly contributed to the eventual Allied victory in the war. The restricted supply of strategic materials such as metal ores and oil had a detrimental effect on the Central Powers' war effort, despite ingenious efforts to find other sources or substitutes.

Through a sequence of events, the Allies declared foodstuffs contraband, and this aspect of the blockade remains most controversial. In December 1918, the German Board of Public Health claimed that 763,000 German civilians had already died from starvation and disease caused by the blockade.C. Paul Vincent, The Politics of Hunger: the Allied Blockade of Germany, 1915–1919. Athens, Ohio: Ohio University Press, 1985. p. 145"Schädigung der deutschen Volkskraft durch die feindliche Blockade" [Damage to the German National Strength by the Enemy Blockade]. Memorandum of the (Reich Board of Health), 27 December 1918. Berlin: Reichsdruckerei. The report notes on page 17 that the figures for the second half of 1918 were estimated based on the first half of 1918. An academic study done in 1928 put the death toll at 424,000,Grebler, Leo (1940). The Cost of the World War to Germany and Austria–Hungary. Yale University Press, 1940, page 78. with similar or lower numbers given by more recent scholars, noting however complications with the degree of attribution of deaths to the Spanish flu of 1918-1920. Around 100,000 people may have died during the post-armistice continuation of the blockade into 1919.The Blockade of Germany after the Armistice 1918–1919 Bane, S. L. 1942 Stanford University Press, page 791 However, it has been pointed out that there was an even slightly larger civilian excess mortality during the war in the United Kingdom and France, both countries that were much less affected by food shortages (although this can also be attributed to the influenza epidemic and to diseases such as bronchitis and tuberculosis which are not strictly nutrition-related).

Germany, France and the United Kingdom relied heavily on imports to feed their populations and to supply their war industry. Imports of foodstuffs and war materiel to the European belligerents came primarily from the Americas and had to be shipped across the Atlantic Ocean, which impelled Britain, France and Germany to aim to blockade each other. The British Royal Navy, combined with the French Navy, out-numbered the Central Powers' ships and could operate throughout the British Empire and the French colonial empire, while the German , its surface fleet mainly restricted to the German Bight, used its commerce raiders and submarines elsewhere. Germany was initially able to use neutral countries as a conduit for international trade, but eventually British and French pressure, American involvement, and German missteps led to the full economic isolation of Germany.

Background In April 1904, the UK and France signed a series of agreements and improved their relations (Entente Cordiale). Prior to World War I, they held a series of conferences at Whitehall in 1905–1906 concerning military co-operation 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 act as an economic weapon to destroy German commerce. It was not until 1908, however, that a blockade of Germany formally appeared in war plans of the Royal Navy 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, with the navy undecided over how best to operate such a blockade. A traditional "close blockade" involved warships being stationed directly outside an enemy's ports. By 1912, improvements in naval technology especially in torpedo boats, mines, coastal artillery and submarines led to fears that blockading ships would be vulnerable. Therefore, by July 1914, the British had decided that in the event of war with Germany, a "distant blockade" controlling entry to the Atlantic Ocean through the North Sea would be the best strategy.

Meanwhile, the German Empire had not made specific plans to manage its wartime food supplies since in peacetime; it produced about 80% of its total consumption. The Germans also expected to requisition supplies from occupied territories, furthermore, overland imports from the Netherlands, Scandinavia and Romania would be unaffected by any naval blockade. A key component of German military thinking was the realization that notwithstanding food supplies, Germany's prospect of winning a long war with relatively weak allies against the United Kingdom, France and Russia was dubious in any case. The Schlieffen Plan was the product of this mindset, and had left the General Staff confident that the war would be over (at least in the west) long before food shortages might otherwise have become an issue. However, once it became clear that the Schlieffen Plan had failed and that Germany would have to fight a long war on two fronts, factors such as the conscription of farm laborers, the requisition of horses, poor weather and the diversion of nitrogen from fertilizer manufacture into military explosives all combined to cause a considerable drop in agricultural output.

A key factor was the 1909 Declaration of London, which attempted to establish the generally recognized rules of international law. While signed, it was never formally ratified by any country (the US Senate consented to, not in time for the start of the war). The British, in particular, did not wholly accept the Declaration, but did not disregard it entirely either. As well as specifying certain rules on the treatment of neutral ships, the declaration defined three categories of neutral cargo during war:

Absolute contraband, which is clearly military cargo that can be seized without notice. Conditional contraband, which are dual purpose goods including foodstuffs. These could be captured if "shown to be destined for the use of the armed forces or of a government department of the enemy state". Goods not to be declared contraband, such as medical supplies, but also certain civilian raw materials and goods.

The British would "modify" and "supplement" the Declaration in their blockade.

Blockade

The United Kingdom, with its overwhelming sea power, established a naval blockade of Germany immediately on the outbreak of war in August 1914. This was strengthened or weakened in a number of steps.

20 August 1914, a Maritime Order in Council declared that Conditional Contraband would be treated as Absolute Contraband. 21 September 1914, the Contraband Proclamation reassigned many goods from the "not to be declared contraband" list to the Conditional Contraband list. 29 October 1914, due to American protests, a new Maritime Order in Council repealed the 20 August order, but put the onus on the owners of the goods to prove there was not a military destination. 2 November 1914, accusing Germany of illegally placing naval mines, Britain declared the North Sea a "military area". This meant that to ensure "commerce of all countries will be able to reach its destination in safety", traffic through the area was recommended to follow specific lanes (to avoid German mines and British mines, ostensibly placed to protect against German warships), forcing them to submit to British inspection. 11 March 1915, a Maritime Order in Council announced that the British would "seize all ships carrying goods of presumed enemy destination, ownership, or origin". This was in retaliation for the February 1915 German "war zone" announcementUnlike the British November declaration, this was a pseudo-unrestricted submarine warfare campaign, where ships were liable to be sunk without warning.

The last of those measures was also preceded by the case of the , a US-flagged cargo ship carrying American foodstuffs to Hamburg. The owners of the cargo had issued a guarantee that the food would not be used by the German military, which was accepted by the British. However, while the ship was en route, on 26 January 1915, the German Federal Council announced that the German government would seize all grain in Germany, a decree interpreted by the British as putting the food supply all under the control of the German Army. Thus the Wilhelmina was detained in Falmouth, Cornwall from 9 February, while the British, Germans and Americans debated how the decree affected the original assurance. The 11 March Order in Council overtook prize court proceedings which were originally planned for 31 March and the issue was eventually settled in July 1916 by Lord Mersey directing the British Government to purchase the cargo and pay damages to the ship owners, rejecting the Americans' claims of neutral trading rights.

Eventually, it became clear that the British measures all but prevented maritime neutral trade, including foodstuffs, with the Central Powers. While the British avoided the use of the word "blockade" in the above pronouncements, their actions presented an effective "distant blockade", in direct contravention of much of the London Declaration. The British defended their actions by pointing out that they had never ratified the agreement, by arguing that they were retaliating for German actions, by suggesting the Declaration failed to anticipate the military use of some goods (such as rubber), and by referring to the German legal argument that coastal towns could be treated as fortifications and subjected to bombardment. Despite protests, 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.

In March 1916, at the suggestion of Robert Peet Skinner, the United States Consul General in London, the inspection procedure was considerably simplified for neutral shipping. A certified manifest could be sent in advance by telegram to the local British embassy, which, if agreed, could issue a document known as a "navicert", which was forwarded to the Admiralty and would allow the cargo to pass through the blockade without the need for inspection.

The British Isles formed what Admiral Beatty described as "a great breakwater across German waters thereby limiting the passage of vessels to the outer seas to two exits". The Dover Patrol closed off the narrow English Channel, while the Northern Patrol closed the North Sea across the gap between Shetland and Norway, supported by the huge Grand Fleet based at Scapa Flow in Orkney. A British submarine flotilla operated in the Baltic Sea to impede the supply of Swedish iron ore to Germany. A memorandum to the British War Cabinet on 1 January 1917 stated that very few supplies were reaching Germany or its allies via the North Sea or other areas such as Austria-Hungary's Adriatic ports, which had been subject to a French blockade since 1914.

The German government regarded the blockade as an attempt to starve the country into defeat. Militarily, they attempted to retaliate in kind, in particular through the U-boat campaign. The German High Seas Fleet also set out multiple times from 1914 to 1916 to reduce the British Grand Fleet and regain access to vital imports. The sea conflicts culminated in the indecisive Battle of Jutland in 1916, never succeeding in breaking the blockade.