Although Posthumus was himself a social democrat, he was aware from the outset that social order models besides socialism merited study as well. His collection efforts reflected ideological neutrality, and he was driven exclusively by intellectual curiosity – a position that the IISH maintains to this day. This explains how the Institute became the most important repository in the world on the history of anarchism. The acquisition of the vast Max Nettlau collection was crucial in this respect. Posthumus had difficulty convincing the great entrepreneurs on the board of the NEHA of its importance, and this may have helped justify establishing a new, international institute. Nettlau’s collection also comprised the papers of many famous anarchists, including those of the Russian Mikhail Bakunin (40-41). This motivated other anarchists, such as Emma Goldman (42-45) and Aleksandr Berkman, to entrust their papers to the IISH as well.

Anarchism is a highly variegated phenomenon, and all important anarchist movements are represented in the collection of the IISH. These include ‘propagandists of the deed,’ who held that bombs were the most useful arms in the class struggle (44); the peaceful founders of egalitarian communes (45); small groups of intellectuals and artists from countries such as China and Japan (46); as well as the anarchist mass movements that have existed in Russia, Italy, and especially the Spanish-speaking world. Anarchists and anarcho-syndicalists figured very prominently in Spanish politics and society from the late nineteenth century, until they encountered opposition from both Franco’s fascists and the communists in the Spanish Civil War (47). In Latin America as well, especially in Argentina, the anarchist movement was a leading force for half a century or more from 1880 (48).