The article examines the relationship between the dealer Léonce Rosenberg and the artist Gino Severini between 1917 and 1921. The mutual exchange between the two was not simply commercial but also intellectual and personal. The artist helped the dealer to create an international network while World War I was yet to come to an end and before Rosenberg could actually open his "Galerie de L'Effort Moderne". In exchange the dealer encouraged Severini's interest in art theories fostering the initial conception of his book "Du Cubisme au Classicisme" and supported the new direction his painting was taking in terms of style and subject matter. The international dimension of the artistic and commercial network they were part of is confirmed by the commission from the English family of the Sitwells which Severini received through Rosenberg.