Léonce Rosenberg was a Paris-based dealer and major promoter of contemporary French art during the period between the two World Wars. Along with his brother, Paul, he took control of his father's gallery in 1906, but the business split four years later. An enthusiastic collector, Rosenberg had an eye for contemporary aesthetic trends, purchasing work from Picasso and Rousseau, as well as examples of Asian, Egyptian and African art.
Rosenberg stepped in and signed under contract Braque, Gris and Léger when their regular dealer, German-born David-Henri Kahnweiler was forced to flee during WWI. With such artists under contract he opened the Galerie de l'Effort Moderne in 1918, initiating a series of amibitious exhibitions. But within several years, a large number of his Cubist circle had defected to other dealers.
In 1921, Rosenberg cooperated with the post-war liquidation of the Galerie Kahnweiler, an unpopular move that alienated him from many artists. He published 40 issues of the "Bulletin de l'Effort Moderne," relentlessly espousing his own vision of Cubism and providing a platform for the artistic theories of De Chirico, Savinio and Severini. The gallery was badly hit by the Depression, and was unable to recover.