The SFIO was part of France's tripartisme government from 1944 to 1947, but after the war faced a resurgent Communist Party, which achieved a higher share of the vote in every election for the next three decades.

Three years later, Jules Guesde and Paul Lafargue (the son-in-law of Karl Marx) left the federation, which they considered too moderate, and founded the French Workers' Party (POF).

Consequently, the function of secretary general, held by Louis Dubreuilh until 1918, was essentially administrative and the real political leader was Jean Jaurès, president of the parliamentary group and director of L'Humanité, the party's newspaper, Unlike the PRS, SFIO members did not participate in Left Bloc governments, although they supported a part of its policy, notably the laïcité, based on the 1905 Act of separation between church and state.

The assassination of Jaurès on 31 July 1914 was a setback for the pacifist wing of the party and contributed to the massive increase in support for the wartime government of national unity.

Paul Faure became secretary general of the SFIO, but its most influential figure was Blum, leader of the parliamentary group and director of a new party paper Le Populaire.

Those who became Communists created the Confédération générale du travail unitaire (United General Confederation of Labour; CGTU) which fused again with the CGT in 1936 during the Popular Front government.

Léon Jouhaux was the CGT's main leader until 1947 and the new split leading to the creation of the reformist union confederation Workers' Force (CGT-FO).

Édouard Herriot, Paul Painlevé and Aristide Briand succeeded each other as prime minister until 1926, when the French right came back to power with Raymond Poincaré.

Following 6 February 1934 crisis, which the whole of the socialist movement saw as a fascist conspiracy to overthrow the Republic, a goal pursued by the royalist Action Française and other far-right leagues, anti-fascist organisations were created.

The Trotskyist leaders of the Communist League (the French section of the International Left Opposition) were divided over the issue of entering the SFIO.

The Popular Front strategy was adopted in the 1936 French legislative election and the coalition gained a majority, with SFIO obtaining for the first time more votes and seats than the Radical Party.

"), but Pivert would later split and create the Workers and Peasants' Socialist Party (PSOP), with historian Daniel Guérin also being a member of the latter.

The demoralised left fell apart and was unable to resist the collapse of the Third Republic after the fall of France in the military defeat of 1940 during World War II.

Pierre Fourcaud created with Félix Gouin the Brutus Network in which Gaston Defferre, later mayor of Marseilles for years, participated along with Daniel Mayer.

At the same time, Marcel Déat and some neosocialists who had split from the SFIO in 1933, participated to the Vichy regime and supported Pétain's policy of collaboration.

After the liberation of France in 1944, the PCF became the largest left-wing party and the project to create a labour-based political party rallying the non-Communist Resistance failed in part due to the disagreements opposing notably the Socialists and the Christian Democrats about laïcité and the conflict with Charles de Gaulle about the new organisation of the institutions (parliamentary system or presidential government).

This coalition led the social policy inspired by National Council of Resistance's programme, installing the main elements of the French welfare state, nationalising banks and some industrial companies.

While serving in government during the Forties, the SFIO was partly responsible for setting up the welfare state institutions of the Liberation period and helping to bring about France's economic recovery.

[8] In May 1946, the Socialist-led government of Félix Gouin passed a law that generalised social security, making it obligatory for the whole population.

[9] A number of progressive reforms were also introduced during Paul Ramadier's tenure as prime minister in 1947, including the extension of social security to government workers[10] the introduction of a national minimum wage[11][12] and the granting from April 1947 onwards of allowances to all aged persons in need.

An Order of July 1947 prescribed the installation of showers for the use of staff "employed on dirty or unhealthy work" and a decree of August 1947 indicated the special precautions to be taken "to protect workers spraying paint or varnish".

Through the efforts of the SFIO, a comprehensive Farm Law was passed in 1946 which provided that sharecroppers had the right to renew their options at the expiration of their leaseholds and that the owner could repossess the land only if he or his children worked it.

The sharecroppers also had the right to join a marketing cooperative, while their conflicts with owners were to be resolved at arbitration tribunals to which both sides elected an equal number of representatives.

This was relatively weakened by the 1948 creation of a social-democratic trade union Workers' Force (FO) which was supported by the American Central Intelligence Agency.

A Third Force coalition was constituted by centre-right and centre-left parties, including the SFIO, in order to block the opposition of the Communists on the one hand, and of the Gaullists on the other.

At the beginning of the 1950s, the disagreements with its governmental partners about denominational schools and the colonial problem explained a more critical attitude of the SFIO membership.

During the 1956 French legislative election campaign, the party took part in the Republican Front, a centre-left coalition led by Radical Pierre Mendès France, who advocated a peaceful resolution of the conflict.

The French Fifth Republic's constitution had been tailored by Charles de Gaulle to satisfy his needs and his Gaullism managed to gather enough people from the left and the right to govern without the other parties' help.

The SFIO and the Radicals then created the Federation of the Democratic and Socialist Left (FGDS), a centre-left coalition led by Mitterrand.

The SFIO suffered a split in Senegal in 1934 as Lamine Guèye broke away and formed the Senegalese Socialist Party (PSS).