This study uses qualitative data on the trajectories of French military officers to provide preliminary hypotheses on the internationalisation of military careers and the dynamics of international military socialisation. It is divided into three sections. The first section provides an overview of the structure of the French armed forces and gives details on the biographical qualitative methods used throughout the article. The second and third sections describe the types of internationalisation that occur during the first and second phases of military careers respectively. The article mainly contends that French officers are unprepared for the type of internationalisation they experience in the framework of European security institutions. In spite of prior experiences of international contexts, they are forced to learn most of their work on the job and to improvise in their handling of international interactions and negotiations. These findings are shown to have implications for debates in the sociology of professions and the study of European security integration.