“Collective security”, an expression that emerged in the 1930s, is an attempt to respond to the outbreak of violence in the two world wars of the twentieth century. In contrast to the notion of security through a balance of power, which characterised the international system in the nineteenth century, collective security instead relies on the “imbalance of power” (Marie-Claude Smouts and Guillaume Devin) that would be amassed by all member states against any aggressor. This system was first institutionalised in the aftermath of the First World War by the League of Nations and taken up again in 1945 by the United Nations. While never bringing the results hoped for by its advocates, it nevertheless marks a turning point in the history of international relations.
