This form of international cooperation offered completely new possibilities for the suppression of a system of human trafficking that operated across oceans and continents, but at the same time it conflicted with the interests of particular states and their own mutual rivalries and on several occasions threatened to founder on the limitations imposed by national sovereign rights. Alongside the viability of the agreed measure, then, Chapter 6 looks at the diplomatic wrangling by which the British government tried to secure treaty obligations from as many states as possible and to overcome massive political resistance, notably from Spain, Portugal, Brazil, France and the United States. Yet this decades-long process of negotiation produced a mounting international consensus, particularly from the mid-1840s onwards, condemning slavery as a ‘crime against humanity’. One telling sign of this new moral climate was the emergence of one of the first international treaty regimes, which extended from Europe across North and South America and the Arab World to East and West Africa. Its foundational idea was to enforce an agreed humanitarian norms by military means if necessary. The fight against the slave trade, it is argued, gave rise to a new conception of intervention, and abolitionism became established as a key international guiding norm for ‘civilisational’ action in the long nineteenth century.

Chapter 5 begins by briefly looking at the British Slave Trade Act of 1807, which also marked the beginning of the Royal Navy’s operations off the coast of West Africa. It concentrates on the developments that led from a national ban and its unilateral military enforcement by the United Kingdom to its international and multilateral implementation. A crucial turning point is marked by the Congress of Vienna, at which the political pressure built up by the abolitionists was so great that, for the first time, the proscription of the slave trade was jointly proclaimed and enshrined as a humanitarian norm in international law. This interdict then formed the point of reference for a series of highly controversial negotiations between the European states to decide on collective measures to be taken against this border-crossing problem. A bilateral approach between Britain and the continental powers finally resulted in a mechanism for implementation to be set up, which consisted of a previously unheard of combination of military and legal measures and which, in the shape of the Mixed Commissions for the Abolition of the Slave Trade, produced one of the first forms of international jurisdiction.