This chapter tackles the question whether a subsidiarity/complementarity principle – as is set out in Article 17 of the Rome Statute of the International Criminal Court – governs domestic prosecutions for international crimes on the basis of universal jurisdiction (a ‘horizontal’ complementarity principle): are ‘bystander’ states that have no link with those crimes but nevertheless want to prosecute them required to defer to states that have a stronger link and that are able and willing to investigate and prosecute? The author derives from a discussion of six relevant elements of the international legal system that there is no strong legal requirement of horizontal complementarity. Nevertheless, it amounts to proper criminal policy to defer to ‘territorial’ states (states on the territory of which the crime has been committed) that intend to embark in good faith on their own investigations and prosecutions.

Introduction

The complementarity principle was established by the drafters of the Rome Statute. As such, it was designed for vertical application. Indeed, a supranational institution, the International Criminal Court (‘ICC’), would supervise the investigative and prosecutorial work of states, and assume its responsibilities if that work proved to be below acceptable standards. So far, however, scant attention has been paid to the horizontal dimension of complementarity. Horizontal complementarity, the term used in this contribution, refers to the complementary prosecutorial role played by ‘bystander’ states, i.e. those states that do not have a strong nexus with an international crime situation (and that are, for instance, exercising universal jurisdiction), vis-à-vis states that are directly concerned with such a situation, e.g. because the situation occurred on their territory or because the crimes were perpetrated by their nationals (hereinafter denoted as ‘the territorial/national state’ or, generically, as ‘the home state’).