We remember months, because months have names, because they are linked to the seasons, and because they have their own character. Looking at the First World War month by month reveals its complexity while preserving a sense of time. From the opening shots to the signing of the armistice, the First World War lasted almost 52 months. It was fought on land, sea and in the air. It became industrial, and unrestricted: poison gas, aerial bombing of cities, and the sinking without warning of merchantmen and passenger ships by submarines. Casualties, military and civilian, probably exceeded 40 million. Four empires collapsed during the course of the war – the German, Austro-Hungarian, Russian and Ottoman.