Britain declared war on Germany on 4 August 1914, after the invasion of Belgium, whose neutrality Britain had guaranteed. Around 900,000 British and Empire servicemen were dead by November 1918.
The scale
The numbers are difficult to hold. Roughly one in eight British men who served was killed. Around 1.6 million more were wounded, many permanently disabled at a time when there was no adequate provision for them.
Almost every town and village in Britain has a war memorial, and a great many list the names of a substantial fraction of the men of one generation. The handful of “Thankful Villages” — around fifty, out of some 16,000 parishes — are those where every man who went to war came home. They are noted precisely because they are so rare.
The Pals battalions
A recruiting idea with a terrible consequence. Men were encouraged to enlist with friends, workmates and neighbours on the promise of serving together, and whole battalions were raised from single towns, factories, football clubs and offices.
It worked as recruitment. It meant that when a battalion was destroyed, the casualties fell on one street, one workplace, one town in a single morning. Accrington, Leeds, Sheffield, Barnsley and Bradford all lost the greater part of a generation of young men in hours.
The Somme
On 1 July 1916, the first day of the Battle of the Somme, the British Army suffered 57,470 casualties, of whom 19,240 were killed. It is the worst day in the history of the British Army and remains so by an enormous margin.
The battle continued for four and a half months for an advance of about six miles.
The other fronts
The Western Front dominates British memory, but the war was fought at Gallipoli, where a campaign intended to knock Turkey out of the war failed at enormous cost to British, Australian and New Zealand troops; in Mesopotamia and Palestine; in East Africa; and at sea.
Around 1.5 million men from India served, and substantial forces came from Canada, Australia, New Zealand, South Africa, the Caribbean and Africa. The Commonwealth contribution was for a long time under-represented in British commemoration, and correcting that has been a significant recent effort.
The home front
Conscription was introduced in 1916, the first time in British history. Around 16,000 men registered as conscientious objectors and were treated harshly, some imprisoned. Women entered munitions factories, transport and agriculture in enormous numbers, and the war is generally credited with accelerating the extension of the vote to women in 1918.
What it left
The war created the modern British architecture of remembrance: the Cenotaph, the poppy, the two-minute silence, the Tomb of the Unknown Warrior in Westminster Abbey, and the Commonwealth War Graves Commission’s principle that every man be commemorated by name regardless of rank.
It also left a lasting cultural reading of the war as futile slaughter directed by incompetents — shaped by the war poets, by Oh! What a Lovely War and by Blackadder Goes Forth. Historians have argued for decades that this is unfair to the learning curve of the British Army, and the popular version has proved entirely immovable.