Many of the tactics, rhetoric, and strategies of reform activists who, around the turn of the century, worked to pressure the Belgian colonial authorities to better treat their Congolese subjects were direct outgrowths of earlier movements against slavery, inhumane execution, and torture. Like these earlier activists, many of the reformers were missionaries (e.g. George Washington Williams and William Henry Sheppard) or petty government officials (e.g. Roger Casement) whose work allowed them to travel widely. This allowed these reformers to bring back journalistic, often highly detailed accounts of what was going on in the Congo to sympathetic, liberal, middle-class audiences in other parts of the world. This approach was very similar to abolitionists like William Lloyd Garrison, who used newspapers and traveling lecturers to inform affluent Northern whites about the brutality of slavery in the South. This helps to explain another similarity: the appeal to moralistic, religious rhetoric on the part of reformers (as opposed to, say, economic arguments). The strategy of moral persuasion put these reform movements in direct contrast to other burgeoning social movements of the day, such as trade unionism, that used direct pressure tactics like strikes and boycotts to force the hands of decision-makers rather than appealing to their better angels.
There were, however, at least two important differences between these reform activists and their predecessors. One is the international scope and internationalist strategy that formed their work. Unlike anti-slavery activists, who usually appealed directly to their own governments to abolish the slave trade and later slavery itself (e.g. Wilberforce in England and Garrison in the United States), reform activists concerned about the Congo had little success in appealing to the Belgians. Rather, their strategy was to pressure other governments (such as those of the UK and the US) to pressure Belgium to institute reforms. This is part of what made the Congo struggle one of the first truly modern human rights struggles in history. In this way, it bears more similarity to the movement a century later to free prisoners of conscience in the Soviet Union or provoke international humanitarian intervention in Liberia, Sierra Leone, or Darfur than it does to earlier struggles.
The second major difference was the movement's reliance upon modern technology, particularly photography. As Hochschild notes of one of the most prominent reformers, ED Morel,
A master of the media of his day, Morel made particularly effective use of photography. A central part of almost every Congo protest meeting was a slide show, comprising some sixty vivid photos of life under Leopold's rule; half a dozen of them showed mutilated Africans or their cut-off hands. The pictures, ultimately seen in meetings and the press by millions of people, provided evidence that no propaganda could refute.
Where the abolitionists and other earlier reformers had relied upon newspapers, reform activists concerned with the Congo relied upon photography, the (relatively) new mass media, in the same way that contemporary activists rely upon social media. The principle was the same: provoke moral outrage by furnishing people with the horrific details of what is happening in hopes that this will prod them into acting differently. But new technology allowed a new tactic to emerge to accomplish this goal.