The blockade of Germany, during World War I, was a part of the First Battle of the Atlantic between the United Kingdom and Germany. 

The British established a naval blockade of Germany early in the war. As shown later in the Battle of Jutland, the German fleet could not defeat the British fleet to break the blockade. This blockade was unusually restrictive in that even food was stopped as it was said to help the war. The Germans regarded this as an attempt to starve the German people into submission and wanted to fight back. They blockaded Britain and France.

As Germany could not fight with the big British Royal Navy on an even basis, the only possible way Germany could impose a blockade on Britain was through the submarines. The Chancellor of Germany was against this sort of blockade because it meant attacking neutral ships as those of the United States as well. But the military pushed unlimited submarine warfare forward.

On 4 February 1915, Kaiser Wilhelm II of Germany declared the seas around the British Isles a war zone. Effective 18 February, Allied ships in the area would be sunk without warning. British ships hiding behind neutral flags would not be spared, though some effort would be made to avoid sinking clearly neutral vessels.

Up to 750,000 civilians may have died because of starvation caused by this blockade during the War. Many more had to die from starvation after the Armistice in November 1918 as the blockade was continued into 1919, in order to force Germany to sign the Treaty of Versailles in June 1919.

Fighting 
The German U-boat force was now primarily based at Ostend in Belgium. So German submarines had better access to the sea lanes around England. The Germans made use of this advantage and sent out about 20 U-boats to begin the naval blockade. In January, before the declaration of "unrestricted submarine warfare" as the submarine blockade was called, 43,550 tonnes of shipping had been sunk by U-boats. The number of sinkings then steadily increased, with 168,200 tonnes going down in August.

Losses of British warships were small. Although the battleship Formidable had been sunk by U-24 on New Year's Day, the fast destroyer screens soon made successful attacks on battleships and cruisers a thing of the past. On the other hand, there was little a Royal Navy warship could do to sink a U-boat if the submarine's captain was reasonably alert. The U-boat was generally safe from shelling once it had submerged. It could be rammed if it were at periscope depth, but ramming was hardly a reasonable tactic as a standard practice.

Destroyers were not able to hunt the U-boats as they were protecting the fleet, so the British pressed every vessel they could into service, including yachts and trawlers, as auxiliary patrol vessels. However, the U-boats were able to easily evade the patrols and sink merchant vessels traveling unescorted.

Militarily, unrestricted submarine warfare was proving a great success, and the U-boats stood a good chance of starving Britain into surrender. However, in terms of the propaganda war, it was a great disaster for Germany. America wanted to stay out of the European war, but American public opinion had turned against Germany, as unrestricted submarine warfare seemed to confirm the German reputation for brutality. The deaths of American citizens traveling on British vessels torpedoed by U-boats began to make headlines in the US.

When on 7 May 1915, the liner RMS Lusitania was sunk by a German submarine, American outrage brought the US closer to declaring war on Germany.  Under threat of US retaliation, on 27 August, the Kaiser imposed severe restrictions on U-boats attacks against large passenger vessels. On 18 September 1915, he called off unrestricted submarine warfare completely.

Under military pressure on Germany early in 1917, the Kaiser declared full unrestricted submarine warfare once more. Some German diplomats believed that the Americans would avoid war at all costs, and if not, hopefully Germany could bring Britain to its knees before American power made much difference in Europe. In February, 86 vessels were sunk, followed by 103 in March, and then 155 in April. The US finally declared war on Germany in April 1917, over the Zimmermann Telegram. America and Britain together were able to defeat the U-boats.

References 

World War I