Part III discusses the striking triangular relationship between humanitarianism, interventionism, and colonialism and imperialism in various parts of the world. It asks to what extent the idea of humanitarian intervention solidified in international politics as a colonial and imperial practice. Indeed, Chapter 7 will show how closely the struggle against the slave trade was intertwined with the colonial and imperial penetration of Africa. In West Africa British anti-slavery measures, which for strategic reasons increasingly shifted from seaborne military operations to dry land, led to direct interference in the internal affairs of African principalities. A particularly prominent case was Lagos, which ended up being formally annexed by the United Kingdom. From the middle of the century onwards, the by now tried and tested intervention measures came to serve as an example for the suppression of the slave trade in East Africa, increasingly turning the idea of abolition into a decisive catalyst and trailblazer for European expansionism across the African continent. At two international conferences – first in Berlin (1884–85) and then in Brussels (1889–90) – the ‘civilised’ states signed treaties by which they gave themselves a mandate in international law and thereby an effective carte blanche for direct intervention, in the name of civilisation, in the internal affairs of African realms.

Ambassadorial conferences formed a primary mechanism of the Vienna system of international relations and of the related European security culture that emerged after 1815. These gatherings offered more flexible opportunities for multilateral consultation and negotiation than did the rarer congress summit meetings. The London conferences of 1816–19 were the first to be planned, as part of British Foreign Secretary Castlereagh's efforts to internationalise abolition of the African slave trade, and they ultimately also took up interdiction of the Barbary corsairs of North Africa. The conferences established connections between these issues that are crucial to understanding European policies toward both abolition and the corsairs, and which reveal how these questions were matters of European – and African – security as well as of humanitarian intervention. As the Vienna settlement extended beyond Europe into the Atlantic and Islamic worlds, the projection of European power overseas to protect security of persons and property could at the same time bring violence in its wake.