The International Workingmen's Association (IWA) (1864–1876), sometimes called the First International, was an international organization which aimed at uniting a variety of different left-wing socialist, communist and anarchist political groups and trade union organizations that were based on the working class and class struggle. It was founded in 1864 in a workmen's meeting held in Saint Martin's Hall, London. Its first congress was held in 1866 in Geneva.
In Europe, a period of harsh reaction followed the widespread Revolution of 1848. The next major phase of revolutionary activity began almost twenty
years later with the founding of the IWA in 1864. At its peak, the IWA
had 5 million members according to the police reports, although the official journal reported 8 million members. (Source: Wikipedia)