The Tree of Heaven by May Sinclair

The young men had gone -- Morton Ellis, who had said he was damned if he'd fight for his country; and Austin Mitchell who had said he hadn't got a country; and Monier-Owen, who had said that England was not a country you could fight for. George Wadham had gone long ago. That, Michael said, was to be expected. Even a weak gust could sweep young Wadham off his feet -- and he had been fairly carried away. He could no more resist the vortex of the War than he could resist the vortex of the arts.

Michael had two pitiful memories of the boy: one of young Wadham swaggering into Stephen's room in uniform (the first time he had it on), flushed and pleased with himself and talking excitedly about the "Great Game"; and one of young Wadham returned from the Front, mature and hard, not talking about the "Great Game" at all, and wincing palpably when other people talked; a young Wadham who, they said, ought to be arrested under the Defence of the Realm Act as a quencher of war-enthusiasms.