L'union sacrée (French for Sacred Union) was a political truce in France in which the left-wing agreed, during World War I, not to oppose the government or to call any strike. Made in the name of patriotism, it opposed the French Section of the Workers' International's (SFIO) internationalism and its late former leader Jean Jaurès' pledge not to enter any "bourgeois war." Although an important part of the socialist movement joined the Union sacrée, some trade unionists such as Pierre Monatte opposed it.
On the first of August, 1914, the President of the French Republic, Raymond Poincaré, declared war on Germany. Three days later, Prime Minister Rene Viviani made a speech:
This political movement may have been an attempt to create solidarity during a time when the largely pacifist French Socialist Party threatened a general strike, while many French Catholics felt slighted by a 1905 law separating Church and State. Elements of nationalism, the fact that the Germans attacked rather than the French, anti-German propaganda and a desire to regain the former French territory of Alsace-Moselle may have provided further impetus for the movement.