The rope is cut
On the 2nd of August 1914, France mobilised, and Braque was called up that same day; Léger went into the army that month; Apollinaire, Italian-born and not yet a French citizen, talked his way into uniform by the end of the year. Picasso — a Spaniard, and neutral — was not called, and he went down to the station at Avignon to see Braque off. He said afterward that he never saw his old climbing partner again, meaning the partnership: the two of them never again worked side by side. Kahnweiler, a German, had his entire stock of their paintings seized by the French state as enemy property — which left the French government, by accident and decree, holding one of the largest hoards of Cubist art on earth. The laboratory was emptied in a week.
The war did not deal gently with them. Braque took a head wound at Carency in 1915, was trepanned (a disc of skull cut away to ease the pressure), and came back a slower, graver painter. Apollinaire took shrapnel to the skull in 1916, never fully recovered, and died in the influenza pandemic of November 1918 — two days before the Armistice, at 38. The man who had explained Cubism to the world did not live to see the peace.
A language, loose in the world
Picasso, during and after the war, swerved — into stage designs for the Ballets Russes (the dazzling Paris-based Russian dance company everyone in the arts was chasing), into a cool neoclassicism of heavy, stone-colored figures that looked back to ancient Greece and Rome, and then back into a flat, bright, almost decorative Cubism in big set-pieces like Three Musicians. Juan Gris carried the strict version forward as an almost classical system until his early death in 1927. The movement as a daily shared adventure was over by 1914; but by then it had stopped being a movement and become a language.
That language went everywhere. The Russian Constructivists, the Dutch De Stijl painters, the German Bauhaus, the entire road to pure abstraction — all of them start from the permission Cubism granted: that a picture is a flat made thing, free to show several truths at once and to paste the real world straight into itself. Two painters in Montmartre had spent six years roped together over a single problem; a noisier second wave had argued, written and exhibited it into the open; and between them they handed the rest of the century its grammar.
To watch it all begin in a single object, drop one level down to Les Demoiselles d’Avignon— the canvas that fired the starting gun.