The First World War is not called the Great War for nothing. It was the single most decisive event in modern history, as well as one of the bloodiest: by the time the war ended, some nine million soldiers had been killed. It was also a historical full stop, marking the definitive end of the Victorian era and the advent of a new age of uncertainty. By 1918, the old order had fallen: the Bolsheviks had seized power in Russia; the German, Austro-Hungarian and Ottoman empires had been destroyed; and even the victorious Allied powers had suffered devastating losses. It was supposed to be the war to end all wars. And yet barely two decades later, the world was again plunged into conflict. Little wonder then that historians still cannot agree whether Britain’s engagement was worth it.

For some, the war was a vitally important crusade against Prussian militarism. Had we stayed out, they argue, the result would have been an oppressive German-dominated Europe, leaving the British Empire isolated and doomed to decline. And by fighting to save Belgium, Britain stood up for principle: the right of a small nation to resist its overbearing neighbours.

For others, the war was a catastrophic mistake, fought at a catastrophic human cost. It brought Communism to power in Russia, ripped up the map of Europe and left a festering sense of resentment that would fuel the rise of Nazism. We often forget that, even a few days before Britain entered the war, it seemed likely that we would stay out. H. H. Asquith’s decision to intervene changed the course of history. But was it the right one?