English Channel
First World War

The exceptional strategic importance of the Channel as a tool for blockading was recognised by the First Sea Lord Admiral Fisher in the years before World War I. "Five keys lock up the world! Singapore, the Cape, Alexandria, Gibraltar, Dover." However, on 25 July 1909 Louis Blériot made the first Channel crossing from Calais to Dover in an aeroplane. Blériot's crossing signalled a change in the function of the Channel as a barrier-moat for England against foreign enemies.

Because the Kaiserliche Marine surface fleet could not match the British Grand Fleet, the Germans developed submarine warfare, which was to become a far greater threat to Britain. The Dover Patrol, set up just before the war started, escorted cross-Channel troopships and prevented submarines from sailing in the Channel, obliging them to travel to the Atlantic via the much longer route around Scotland.

On land, the German army attempted to capture French Channel ports in the Race to the Sea but although the trenches are often said to have stretched "from the frontier of Switzerland to the English Channel", they reached the coast at the North Sea. Much of the British war effort in Flanders was a bloody but successful strategy to prevent the Germans reaching the Channel coast.

At the outset of the war, an attempt was made to block the path of U-boats through the Dover Strait with naval minefields. By February 1915, this had been augmented by a 25 kilometres (16 mi) stretch of light steel netting called the Dover Barrage, which it was hoped would ensnare submerged submarines. After initial success, the Germans learned how to pass through the barrage, aided by the unreliability of British mines. On 31 January 1917, the Germans resumed unrestricted submarine warfare leading to dire Admiralty predictions that submarines would defeat Britain by November, the most dangerous situation Britain faced in either world war.

The Battle of Passchendaele in 1917 was fought to reduce the threat by capturing the submarine bases on the Belgian coast, though it was the introduction of convoys and not capture of the bases that averted defeat. In April 1918 the Dover Patrol carried out the Zeebrugge Raid against the U-boat bases. During 1917, the Dover Barrage was re-sited with improved mines and more effective nets, aided by regular patrols by small warships equipped with powerful searchlights. A German attack on these vessels resulted in the Battle of Dover Strait in 1917. A much more ambitious attempt to improve the barrage, by installing eight massive concrete towers across the strait was called the Admiralty M-N Scheme but only two towers were nearing completion at the end of the war and the project was abandoned.

The naval blockade in the Channel and North Sea was one of the decisive factors in the German defeat in 1918.
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