Why did Britain fight in World War I?

Britain entered the First World War on 4 August 1914 because Germany had invaded neutral Belgium, breaking the 1839 Treaty of London under which Britain, along with the other great powers, had guaranteed Belgian independence and neutrality.

The immediate cause was the German Schlieffen Plan. Germany's military strategy for a two-front war against France and Russia required a swift knockout blow against France, achieved by sweeping through neutral Belgium to outflank the French defences. German troops crossed the Belgian border on the morning of 4 August. Britain issued an ultimatum demanding withdrawal by midnight. When no reply came, the United Kingdom declared war.

The deeper causes had been building for decades. A system of rival alliances linked the great powers: Germany, Austria-Hungary, and Italy in the Triple Alliance; France and Russia in a defensive pact; and Britain tied loosely to France and Russia through a series of agreements known as the Triple Entente. Imperial rivalry, a naval arms race between Britain and Germany, and nationalist tensions in the Balkans created an atmosphere in which a local crisis could escalate rapidly.

That crisis came with the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, in Sarajevo on 28 June 1914 by the Serb nationalist Gavrilo Princip. Austria-Hungary issued an ultimatum to Serbia, Russia mobilised in support of Serbia, Germany declared war on Russia and then on France, and the alliance system pulled the remaining powers in. Britain's entry turned a European war into a global one, because the British Empire followed. Canada, Australia, New Zealand, India, South Africa, and the other dominions and colonies all committed troops, making the war a conflict fought on the Western Front, in the Middle East, in Africa, and at sea.

The cost was immense. More than 700,000 British servicemen died, along with hundreds of thousands from the Empire. Major battles at Ypres, the Somme, Passchendaele, and Gallipoli left entire communities bereaved. At home, the war transformed society, bringing women into industry and public service in unprecedented numbers and contributing to the Representation of the People Act 1918, which gave the vote to all men over 21 and to most women over 30. The Armistice of 11 November 1918 ended the fighting, and the Treaty of Versailles of 1919 set the formal terms of peace.

Remembrance of the First World War is marked each year on 11 November, with the red poppy as its symbol and the Cenotaph in Whitehall as its national memorial.