When Germany's ultimatum – delivered, as the Kaiser had explained, ‘only with the friendliest intentions towards Belgium’ – expired in August 1914, von Emmich's infantry crossed the frontier in parade-ground order, accompanied by horse-drawn field kitchens with fires burning and army cooks at attention stirring the regimental stew.

They don't wage war like that any more. Indeed the big wars are rarely now between neighbours: one state will cross a continent or an ocean to attack or invade, or occasionally to defend, another. The choreographed exchange of diplomatic threats, the ultimatum, the formal declaration of a state of war, have been pretty much dispensed with. The last time the United Kingdom declared war was against Bulgaria in 1941.

Among the casualties of warfare during the last hundred years have been many of the rules governing the conduct of hostilities. The Hague and Geneva Conventions describe members of warring states' armies and militias as ‘lawful combatants’.