U-20 German submarine, commanded by Captain Walther Schwieger, that sank the British passenger liner Lusitania off the coast of southern Ireland on 7 May 1915.

Ukraine Territory in SE Europe; part of the Russian empire until March 1917. The collapse of tsarism ushered in a bewildering period of political upheaval and civil war in the Ukraine, during which the capital Kiev changed hands no fewer than 12 times. After the Bolsheviks finally achieved hegemony there, the Ukraine became one of the four founding republics of the Soviet Union in 1922.

Union of Democratic Control (UDC) Organisation of prominent left-wing intellectuals and pacifists, founded in August 1914, that advocated a negotiated end to the war and a transparent foreign policy without secret diplomacy. Its ranks included Norman Angell, Ramsay MacDonald and Bertrand Russell.

Unknown Soldier, Tomb of On Remembrance Day 1920 the body of an unidentified soldier was buried in Westminster Abbey as a memorial to all those who died during the First World War. The inscription includes the words 'They buried him among the kings because he had done good before God and toward his house'.

Unrestricted submarine warfare German policy of indiscriminately attacking Allied and suspicious neutral shipping - both military and merchant - with submarine torpedoes. It was officially resumed, after much hesitation, on 1 February 1917.