The majority of European citizens – often referred to as ‘foreign fighters’ – that have returned to European soil have predominantly been prosecuted for terrorist offences. In recent years several European countries have been prosecuting alleged terrorists cumulatively for both terrorist offences and core international crimes. This paper explores which building blocks need to be in place to allow for such prosecutions. This paper argues that only if a suitable legal and institutional framework is in place can prosecution of core international crimes take place, thereby contributing to achieving accountability of the full range of crimes that have been committed by alleged terrorists in the context of the conflict in Syria and Iraq.