World War I, which began in August 1914, did not immediately bring great changes in Busoni's life. In January 1915 he undertook a long concert tour of America. However, he was unable to return home: during his absence Italy declared war on Germany, and the road to Berlin was closed to the Italian Busoni. Instead, like some other artists (d'Albert, Masereel, Romain Rolland), he went to Switzerland and settled in Zurich for the remainder of the war.

The slaughter into which capitalism dragged humanity shook Busoni profoundly. To his honor, he did not succumb to the lies of the chauvinistic slogans and did not join either imperialist camp. Instead, he found the courage to polemicize openly against the “patriotic ecstasy” that the press of the warring countries attempted to use to silence “that which calls out to Heaven.” The reason for Busoni's writing was the untimely death at the front of one of his friends, the young artist Boccioni. The popular Italian newspaper, Corriere della Sera, had “honored” the memory of the artist with an obituary in which he was portrayed, in direct contradiction to his true views, as a raging nationalist who was happy to give his life for his homeland. In his letter “Der Kriegsfall Boccioni,” published in the Swiss paper Neue Züricher Zeitung, Busoni published excerpts from Boccioni's letters, which proved the opposite, and blamed the prevailing “deliberate misrepresentation of the truth” and “ubiquitous silence about unforgivable events.”