August 1914. German armies cross into Belgium because the war plan cannot wait. Liège's forts, advertised as modern, fold under siege guns built to kill forts. Civilians watch cavalry patrols on the road and think this will be over by harvest. Every capital thinks that.
Why then
Germany's plan for a war on two fronts, named for its author Schlieffen, is a timetable: beat France in six weeks by swinging through Belgium, then turn on Russia. Belgian neutrality rests on an 1839 treaty that Britain signed. Alliances made a war likely; the march through Belgium made it British.
What they wanted
Moltke, the German chief of staff, needs Paris before Russia is ready. Belgium wants to exist. The German chancellor calls the treaty 'a scrap of paper'; Asquith's cabinet decides it is not, and throws an empire into the gears.
What changed
Britain is in, and with it India, Canada, Australia and the rest. German troops shoot some 6,500 Belgian and French civilians in the first weeks, and atrocity stories, true, panicked and invented, become a weapon.
Still argued
For decades 'poor little Belgium' was waved away as British propaganda. When John Horne and Alan Kramer went through the German records, the massacres turned out to be real, even where the stories had grown in the telling.