The foreign terrorist fighter (FTF) phenomenon is high on the agenda of Dutch law and policy makers. Fear of what FTFs might do abroad and upon return has resulted in the increased (call for) criminalization of preparatory conduct (e.g. training, recruitment) with the aim to incapacitate at the earliest possible moment through criminal prosecutions. However, which fundamental criminal law principles limit criminalizing acts, which are not in themselves harmful (e.g. travelling to Syria), but are an expression of the person’s concealed intent to commit a harmful act (e.g. a terrorist attack)? Where on the continuum from a thought to a massacre is the cut-off point that distinguishes legitimate prosecutions from premature ones? This project will provide innovative insights into anticipatory prosecutions regarding the FTF phenomenon, first, by developing a conduct typology, and second, by identifying the (legal) normative framework of justifications (e.g. prevention of harm) and limits (e.g. rule of law-values). A cross-national comparison between cases from the Netherlands – reportedly the first European country to prosecute Islamic State FTFs – and the United States – a nation that has been dealing pro-actively with terrorism issues longer than European countries – will provide the necessary building blocks to do this.