In 1910 industrial workers in Barcelona formed the anarcho-syndicalist trade union, the National Confederation of Trabajo (CNT). Although the CNT operated as a trade union, it also contained subgroups such as Solidarios, a terrorist group led by Buenaventura Durruti.
In 1921 Miguel Primo de Rivera banned the CNT. It now became an underground organization and in 1927 an inner-core of activist established the Federación Anarquista Ibérica (FAI). The FAI was strong in Catalonia and Aragón and members made several unsuccessful attempts to assassinate Alfonso XIII.
The FAI took part in several unsuccessful uprisings including Casas Viejas (January 1933), Saragossa (December 1933) and Asturia (October 1934). It is also estimated that the FAI was responsible for the assassinations of about 150 members of the Falange Española.
In the 1936 Elections the FAI urged anarchists to support the Popular Front in order to defeat the extreme right-wing. After the victory of the Popular Front activists such as Buenaventura Durruti, Federica Montseny and Juan Garcia Oliver helped to establish communes and workers' committees.
On the outbreak of the Spanish Civil War members of the FAI helped to establish the Antifascist Militias Committee in Barcelona. The committee immediately sent Buenaventura Durruti and 3,000 Anarchists to Aragón in an attempt to take the Nationalist held Saragossa. The FAI also saw military action in Catalonia and Valencia.
In September 1936, President Manuel Azaña appointed the left-wing socialist, Francisco Largo Caballero as prime minister. In an attempt to get the full support of the anarchists Largo Caballero brought into his government members of the FAI and CNT, Juan Garcia Oliver (Justice), Juan López (Commerce), Federica Montseny (Health) and Juan Peiró (Industry).
At the beginning of November, 25,000 Nationalist troops under General Jose Varela had reached the western and southern suburbs of Madrid. Five days later he was joined by General Hugo Sperrle and the Condor Legion. This began the siege of Madrid that was to last for nearly three years.
On 14th November Buenaventura Durruti arrived in Madrid from Aragón with 5,000 men. He immediately went to the frontline where the Manzanares River passed through the University of Madrid. On 19th November, 1936, Durruti was shot by a sniper from one of the upper stories of the Hospital Clinic. Durruti died the following day. Durruti's supporters later claimed that he had been killed by a member of the Communist Party.
The CNT and FAI remained strong in Barcelona. In May, 1937, they were attacked by the Assault Guard and the Civil Guard. With the support of the NKVD, the Communist Party now became the dominant force in the city.
The Popular Front government attempted to bring the FAI dominated Anarchist Brigades under the control of the Republican Army. At first the Anarcho-Syndicalists resisted and attempted to retain hegemony over their units. This proved impossible when the government made the decision to only pay and supply militias that subjected themselves to unified command and structure.