French Section of the Workers' International

The French Section of the Workers' International (French: Section française de l'Internationale ouvrière, SFIO) was a political party in France that was founded in 1905 and succeeded in 1969 by the modern-day Socialist Party. The SFIO was founded during the 1905 Globe Congress in Paris as a merger between the French Socialist Party and the Socialist Party of France in order to create the French section of the Second International, designated as the party of the workers' movement.

The SFIO was led by Jules Guesde, Jean Jaurès (who quickly became its most influential figure), Édouard Vaillant and Paul Lafargue (Karl Marx's son in law), and united the Marxist tendency represented by Guesde with the social-democratic tendency represented by Jaurès. The SFIO opposed itself to colonialism and to militarism, although the party abandoned its anti-militarist views and supported the national union government (French: Union nationale) facing Germany's declaration of war on France in World War I, having replaced internationalist class struggle with patriotism like the whole Second International.

Because of conflicting views towards the 1917 Russian Revolution and the Bolshevik-led Communist International, the SFIO split into two groups during the 1920 Tours Congress as the majority created the French Section of the Communist International which joined the Communist International and became the French Communist Party while the minority continued as the SFIO.

Between 1909 and 1920, the SFIO produced L'Humanité. Its national affiliations included the Lefts Cartel (1924–1934), the Popular Front (1936–1938), the Tripartisme (1944–1947) and the Third Force (1947–1958). Internationally, the party was first affiliated to the Second International (1905–1916), then to the Labour and Socialist International (1923–1940) and finally to the Socialist International (1951–1969). The SFIO's symbol was a red and black circle with the Three Arrows.