The necessities of life occupied artists more than ever before. The day before the war broke out, Picasso withdrew all his money from the bank and hid it under his bed. He sent Gertrude Stein a picture postcard of men being marched off to war down the Tuileries and went home to tend his dying girlfriend. A natural pacifist, Picasso kept neutral. He went to Rome, married a dancer and reverted, for the duration, to realism.

For those prepared to serve a cause, there were well-paid stimuli for propaganda art. But when conscientious artists tried to express patriotism the result often backfired. A 1914 violin and piano sonata by Leoš Janáček, supposedly welcoming a war of Slav liberation, struck a morose—even maudlin—note, as did Edward Elgar's 1918 sonata for the same instruments. Freud, had he been asked, might have concluded that these artists had lost control of their unconscious, perhaps also of their libido.

Nevertheless, against the overwhelming impression of collective impotence, something stirred. The writers Stefan Zweig and Romain Rolland founded a peace movement that could have seen them shot for treason. Frontline composers stored abominable artillery noise for future use. Artists reflected and redefined. Modernism, in abeyance, was transformed. No longer a rebellion against a bluff old order, it acquired logic, dignity and a steady, inevitable tread. Modernism would emerge from the war more diffuse but also more powerful, replacing late Romanticism as the mainstream, heralding an era of experiment and anything-goes. Mona Lisa, in 1919, grew a moustache.

One of the most significant byproducts of world war was a globalisation of art. In the trenches, what moved a German artist would also inspire a Russian or a Czech. The writers Joseph Roth on the eastern front and Carl Zuckmayer in the west devoured books in the lull of battle. Zuckmayer was gripped by "a kind of intoxication, an unquenchable thirst for knowledge, culture, insight, learning and understanding . . . I spent a large part of my lieutenant's pay on books . . . I read like a man obsessed."