SFIO leadership
In 1902, Jaurès returned as deputy for Albi. The independent socialists merged with Paul Brousse's "possibilist" (reformist) Federation of the Socialist Workers of France and Jean Allemane's Revolutionary Socialist Workers Party to form the French Socialist Party, of which Jaurès became the leader. They represented a social democratic stance, opposed to Jules Guesde's revolutionary Socialist Party of France.
During the Combes administration his influence secured the coherence of the Radical-Socialist coalition known as the Bloc des gauches, which enacted the 1905 French law on the Separation of the Churches and the State. In 1904, he founded the socialist paper L'Humanité. According to Geoffrey Kurtz, Jaures was "instrumental" in the reforms carried out by the administration, Emile Combes, "influencing the content of legislation and keeping the factions within the Bloc united." Following the Amsterdam Congress of the Second International, the French socialist groups held a Congress at Rouen in March 1905, which resulted in a new consolidation, with the merger of Jaurès's French Socialist Party and Guesde's Socialist Party of France. The new party, headed by Jaurès and Guesde, ceased to co-operate with the Radical groups, and became known as the Parti Socialiste Unifié (PSU, Unified Socialist Party), pledged to advance a collectivist programme. All the socialist movements unified the same year in the French Section of the Workers' International (SFIO).
On 1 May 1905 Jaurès visited a newly formed wine making cooperative in Maraussan. He said the peasants had to unite instead of refusing to help each other. He told them to, "in the vat of the Republic, prepare the wine of the Social Revolution!". As the revolt of the Languedoc winegrowers developed, on 11 June 1907 Jaurès filed a bill with Jules Guesde that proposed nationalization of the wine estates. After troops had shot wine-growing demonstrators later that month, Parliament renewed its confidence in the government. Jaurès's L'Humanité carried the headline, "The House acquits the mass killers of the Midi".
In the general elections of 1906, Jaurès was again elected for the Tarn. His ability was now generally recognized, but the strength of the SFIO still had to reckon with radical Georges Clemenceau, who was able to appeal to his countrymen (in a notable speech in the spring of 1906) to rally to a Radical programme which had no socialist ideas in view, although Clemenceau was sensitive to the conditions of the working class. Clemenceau's image as a strong and practical leader considerably diminished socialist populism. In addition to daily journalistic activity, Jaurès published Les preuves; Affaire Dreyfus (1900); Action socialiste (1899); Études socialistes (1902), and, with other collaborators, Histoire socialiste (1901), etc.
In 1911, he travelled to Lisbon and Buenos Aires. He supported, albeit not without criticisms, the teaching of regional languages, such as Occitan, Basque and Breton, commonly known as "patois", thus opposing, on this issue, traditional Republican Jacobinism. Jaures opposed imperialism, arguing that it posed a threat to peace in Europe.
