Synthesis anarchism, also known as united anarchism, is an organisational principle that seeks unity in diversity, aiming to bring together anarchists of different tendencies into a single federation. Developed mainly by the Russian anarchist Volin and the French anarchist Sébastien Faure, synthesis anarchism was designed to appeal to communists, syndicalists and individualists alike. According to synthesis anarchism, an anarchist federation ought to be heterogeneous and relatively loosely-organised, in order to preserve the individual autonomy of its members.
Since the 1890s, there had been a drive within the anarchist movement to foster cooperation between the various anarchist schools of thought and to unite them across ideological lines. During the dispute between the collectivists and communists, the Italian anarchist Errico Malatesta and the Spanish anarchist Fernando Tarrida del Mármol advocated for anarchists of both tendencies to unite, according to the principle of "anarchism without adjectives".
Over the subsequent decades, Max Nettlau developed this principle into a call for anarchists to unite into a single organisation, which would preserve individual autonomy and encourage mutual aid between its members. The principle of individual autonomy was particularly central to the organisational ideas of Sébastien Faure and Emma Goldman, as well as to the Japanese "pure anarchists" that advocated for a more loosely-organised grouping. The Russian anarchist Volin considered all anarchist schools of thought to be valid, advocating for a pluralistic organisation that unites anarchists of different tendencies behind a set of common principles and goals. While Goldman considered the best path to be a loose framework that extended autonomy to each tendency, Volin and Faure would go on to develop a "synthesis" of the different tendencies, with the aim of creating an organisational platform that would be acceptable to the entire anarchist movement.
Following the October Revolution, many Russian anarchists attempted to escape the Red Terror by fleeing to Ukraine.