The word solidarity is a borrowing from the French solidarité, which until the nineteenth century had the restricted legal meaning of a contractual obligation. I argue that in the pre-revolutionary decades, a newly born French lexeme was much closer to what solidarity would mean for modern societies, at least if we accept the agonistic context of most phenomena of solidarity: ‘esprit de corps’, taken from the military language and changed into a combat concept by the Philosophes. A ‘corps’ in French, among other definitions, is an organised group with its own cohesion, its own interests, and its instinct of preservation. In this article, I argue that the study of the birth of ‘esprit de corps’, and the French Revolution attempt to universalise the concept, can shed some light on later debates on democratic forms of solidarity. If solidarity is a form of esprit de corps, and because of the military origin of the latter, solidarity should be considered as a ‘war machine’, an agonistic union rather than an all-encompassing and vague ideal of global fraternity.