Socialist Party of France (1902)

The merger began when the two parties jointly condemned the entry of the "socialist" minister Alexandre Millerand into the government at the congress of socialist organisations in the Salle Wagram in Paris on 28 September 1900. It took formal shape in November 1901 at the Ivry congress with the creation of the Socialist Party of France, "Revolutionary Socialist Unity", and the following months were spent building unity on the ground. The party was founded at the Commentry congress of September 1902. In March of the same year the "independent" socialists around Jaurès founded their own French Socialist Party at Tours.

The party was led by a fifteen-member Executive Commission of the National Council, which included Jules Guesde, Édouard Vaillant, Bracke, Louis Dubreuilh, Arthur Groussier and Paul Lafargue. Marcel Sembat, a leader of the Revolutionary Socialist Party, moved into it too. Its weekly organ Le Socialiste had been the French Workers' Party's paper. It held congresses at Reims in September 1903, Lille in April 1904 and Paris in April 1905, and in 1904 it claimed 16,000 members. In the municipal elections of 1904 it lost three town halls, Lille, Bourges and Montluçon, but held 59 municipal councils, including Alfortville and Ivry-sur-Seine in the Seine department. At its 1904 congress Vaillant insisted that every socialist deputy had a duty to oppose colonial conquest.

Unlike Jaurès's French Socialist Party, which joined the Bloc des gauches and supported bourgeois governments, it refused such support. Under pressure from the Second International the two parties nevertheless merged in April 1905 into the French Section of the Workers' International, the party standing as one stage in the unification of the French socialists.