The article examines the activity of the pacifist French-language magazine “Demain” (“Tomorrow”), whose editor-in-chief was the famous French publicist Henri Guilbeaux, during World War I. Based on the materials of the investigation and trail of H. Guilbeaux, preserved in the archive of the French Ministry of Defense, the article reconstructs the history of the foundation and publication activity of the magazine in Switzerland in 1916-1919. The pacifist, anti-militarist and revolutionary position of H. Guilbeaux, who left France for neutral Switzerland, which was clearly manifested in the publications of the magazine, gave the French counterintelligence reason to suspect H. Guilbeaux of treason. Special attention is paid to the analysis of the materials of the “Guilbeaux case”, in which counterintelligence officers characterize the main military and political topics that were constantly raised on the pages of the magazine “Demain”, namely: anti-militarism and the appeal for desertion, praise of Germany, accusation of the Entente and, first of all, England, of unleashing World War I, admiration for Russian revolutionaries and the revolution that took place in Russia, as well as the desire to instigate a revolution in France similar to the Russian one. In addition, on the basis of the data collected by French counterintelligence agents, the author lists and characterizes the persons with whom H. Guilbeaux actively communicated and collaborated, especially those who financed the publication and distribution of the magazine “Demain”. The author concludes that by publishing articles by European intellectuals who opposed the war on the pages of his magazine, H. Guilbeaux went beyond his nationalism and became a dangerous pacifist for France. For him, the defeat of France in World War I and the post-war cultural and political rapprochement between France and Germany were preferable.