By the beginning of the 20th century the anarchist movement in Spain was the strongest in Europe. The main support came from the industrial workers of Barcelona and in 1911 activists formed the anarcho-syndicalist trade union, the National Confederation of Trabajo (CNT). This was in response to the execution of the anarchist Francisco Ferrer.

Although the CNT operated as a trade union, it also contained subgroups such as Solidarios, a terrorist group that included men such as Buenaventura Durruti, Francisco Ascaso and Juan Garcia Oliver. Operating between 1920 and 1923 the group assassinated ten public figures.

In 1921 the group were involved in the murder of Eduardo Dato, the Spanish prime minister. Two years later the group assassinated Juan Soldevila Romero, the Archbishop of Sargossa, in revenge for the murder by the police of Salvador Segui, a CNT leader.