This report analyses public-private relations between the European (in particular, French) authorities and Facebook, Google (YouTube) and Twitter on the subject of terrorist usage of social media between 2015 and 2019. In a qualitative approach, interview material is mobilised to investigate lived experiences of this early cooperation, focusing on how stakeholders defined and negotiated their involvement, navigated power relations and pursued strategies to establish working relations without abandoning their respective preferences. The report finds that, although cultural fault lines influenced the stakeholders’ perceptions of roles, enthusiasm for ‘automated’ moderation was shared across the public-private boundary, justifying the development of what can be called state-platform diplomacy.