Defining international terrorism has been an unsolved problem of international law for quite some time. All those who aspire to the promotion of international criminal justice and the fight against impunity agree that the formulation of a universal definition for international terrorism will further enhance the fight against terrorism and offer a universally acceptable legal framework within which this fight can be conducted. In light of this, this thesis is an attempt to approach the issue of defining international terrorism, proposing that the most workable way to this direction is to achieve due balance between the two principle driving forces of international law developments: State sovereignty and cosmopolitan ideals. These dynamics, which often conflict, have been playing a key role in the formation of international law in general and the formulation of definitions for international crimes in particular. As such, the quest for a definition of international terrorism will be based on the argument that its effectiveness relies on the extent to which it manages due balance between these two antithetical poles. As a complement to this argument, the definition of the crime of aggression for the purposes of the Rome Statute will be used as a paradigm of whether and to what extent this desired balance between State sovereignty considerations and cosmopolitan purposes can be achieved and whether there are lessons to be learnt from this process for the purpose of defining international terrorism. It is the author’s view that achieving due balance in formulating a definition for international terrorism can be a realistic prospect, not as a compromise between these two opposing dynamics but as a common effort of the international community to develop international law in this field.