Addressing the question of the movements in the history of ideas that gave rise to such a novel conception of intervention, Chapter 3 considers the intellectual origins of humanitarianism. Beginning in the mid-eighteenth century, various religious, philosophical and literary currents combined to mould a humanitarian sensibility, under the influence of which people developed empathy for their fellow human beings and took active measures to relieve the suffering of others across national and even continental divides. Rather than building their campaigns on a discourse of rights, civil society groups appealed instead to the idea of ‘humanity’, thereby making it a key normative reference in both national and international politics.

Chapter 4 focuses on the struggle against the Atlantic slave trade and the emergence of a humanitarian understanding of intervention. It begins by briefly outlining the system of the transatlantic traffic in slaves, which, by reducing human beings to a mere commodity in a circular trading system, constituted one of history’s worst humanitarian disasters. One of the central concerns of the abolitionists, who towards the end of the eighteenth century grew from a small cohort of well-connected activists to a mass movement, was to reverse this process of dehumanisation and render slaves visible in public discourse as fellow human beings who were suffering and in need of help. The focus is thus placed on the successful humanitarian mobilisation of the public by means of a targeted ‘humanitarian narrative’ and an unprecedented combination of multifarious instruments of appeal. For strategic reasons, the abolitionists concentrated their efforts on the slave trade, which was to be terminated by means of state intervention. A close interlinkage of mobilisation in parliament and civil society can be observed here, for the activists used petitions and legislative initiatives in their attempts to make their cause the official policy of the British government. In doing so, the abolitionists were the first to link humanitarianism with the policy and practice of state intervention.

Chapter 4 examines the influence of the Bryce Group’s war prevention plan more broadly, scrutinising the relationship between the League of Nations Society and its counterpart in the United States, the League to Enforce Peace. While scholars have hardly analysed the two groups’ interactions, both groups sprang from the liberal internationalist tradition, had a lot in common in terms of social background and worked for the same aim of reforming the global order. Such similarities, however, did not enable them to establish a constructive collaboration, let alone a transnational movement. In reality, both groups sought political support for their own post-war schemes and regarded their counterpart merely as a medium for approaching statesmen on the other side of the Atlantic. Moreover, the differences in their domestic contexts and in the British and American liberal internationalist traditions hindered the two groups from building mutual trust and a joint lobbying strategy.