The cross-border agility of European foreign fighters combined with the porosity of SchengenSchengen’s open borders make the foreign fighter conundrum a pan-European issue that calls for transnational cooperation at the European level. This Chapter critically examines the legal response to the foreign fighter phenomenon at the European level in light of the rule of law and human rights reverberations of the legal instrument that most profoundly shaped this response: the EU Directive on combating terrorism. The Chapter sketches the developments that influenced the opaque adoption process of the Directive in the wake of the 2015 Paris Attacks. It outlines how the Additional Protocol to the Council of Europe Convention on the Prevention of Terrorism has introduced an additional layer of treaty obligations to the international legal regime on foreign fighters, suffering greatly from cardinal errors in legal drafting yet shaping the Union’s legal response to foreign fighters. The Chapter identifies several features of the Directive that are disconcerting from a rule of law and human rights perspective: an unnecessarily capacious definition of terrorism that widens the scope of the EU counter-terrorism law in an unsatisfactory fashion, an abundance of ill-defined offences aimed at foreign fighters, and an extraordinary range of ancillary offencesAncillary offences that reach far into the preparatory phase of acts of terrorism and could have rather implausible cumulative effects. Principle of legalityThis definitional imbroglio has resulted in a fragmentary implementation at the domestic level, impeding the development of an effective European criminal justice response to the foreign fighter phenomenon.