1914–1918. The war that was supposed to end war teaches the world to industrialise grief: around twenty million dead, soldiers and civilians, and then a flu, carried home on its troopships, that kills more. The Western Front is the famous scar. It is also Galicia, the Isonzo, the Caucasus, the Tigris, East Africa, and a blockade that starves German cities.
Why then
Alliances, war plans built on railway timetables, and a Balkan murder turn a regional crisis into a continental war in five weeks. Industrial states can then feed millions of men into the same fields for years.
What they wanted
Each cabinet expects a short, limited win. What they get is a war that outruns everyone running it. The empires draw on their colonies: 1.3 million Indian soldiers, African porters by the hundreds of thousands, Chinese labourers digging French trenches.
What changed
When it stops, the German, Austro-Hungarian, Russian and Ottoman empires are gone. The new states, the revolutions, the dictators and the next war are all, one way or another, veterans of 1918.
Still argued
Niall Ferguson argues Britain could have stayed out and left a continental war continental; most historians answer that a quick German victory would have been worse. Keegan, who wrote the battles, called it tragic and unnecessary.