This dissertation is a set of three essays connected by the common theme of solidarity: the way solidarity emerged as a distinct concept and was mobilised for political ends; how its key sociological version was influenced by, and used in, a colonial context; and how liberalism has long worried about solidarity’s social and political uses and how we could respond to this concern. The first essay argues that the primary concept of solidarity proper — Léon Bourgeois’ doctrine of interdependence — was inspired by Louis Pasteur’s microbiological discoveries. I reconstruct and examine Bourgeois’ legalistic doctrine of social debt in the light of the biological episteme of the time, and show how it influenced public policy during the Fin de Siècle. The second essay argues that the common national view of solidarity distorts our understanding: Using the Durkheimian School as a historical lens, I show how this concept of solidarity emerged as tied to colonial ethnographies and reflected a civilisational divide at the turn of the century. I argue that a strand of sociological solidarity influenced French colonial administrators in promoting a developmentalist agenda, as well as international lawyers’ early visions of European unification. The third essay offers a post-foundational response to liberalism’s “fear” of solidarity as perpetually at risk of descending into authoritarian rule. I distinguish a positive and negative concept of solidarity — based on shared identity or in concerted action against adversity — and ground this distinction in a critical reconstruction of Isaiah Berlin and Hannah Arendt’s thought during the decolonization period. I posture that the account developed can avoid this liberal fear and is particularly useful for addressing injustice. The dissertation’s upshot is two-fold. First, if solidarity came to mean different things in its legacy, it was also mobilised for political ends. Second, to fend off its political dangers we have conceptual resources at our disposal which address the idea that solidarity is not about who we are, but what we do together.