Effectuating 'Negative' and 'Positive' Horizontal Complementarity Exploring the Potential of The Ljubljana-The Hague Convention

In its recent ‘Policy on Complementarity and Cooperation’, the Office of the Prosecutor of the ICC has commended the adoption of The Ljubljana–The Hague Convention, stating that ‘(i)t fills a void in the architecture of legal mechanisms designed to promote the investigation and prosecution of core international crimes at the national level’. In light of this statement, this contribution aims to explore what role the Convention can play in effectuating the principle of horizontal complementarity, by developing an inter-State system of cooperation through which domestic justice efforts for core international crimes can be coordinated and supported. Horizontal complementarity is primarily conceived and understood in its negative dimension, which mirrors vertical complementarity under Article 17 of the Rome Statute, and implies that a bystander State’s jurisdiction would be complementary to the jurisdiction of a more closely connected State when investigating and prosecuting core international crimes. In this regard, it will be analysed whether the Convention can contribute to the development of a global system of horizontal complementarity, by examining the Convention’s provisions on jurisdiction and transfer of proceedings. In addition to exploring the Convention’s potential in effectuating this negative dimension of horizontal complementarity, this contribution will analyse whether the Convention could contribute to realizing positive horizontal complementarity, which at the inter-State level could be defined as activities and actions of cooperation aimed at enhancing and enabling States who possess jurisdiction to effectively investigate and prosecute core international crimes cases. In this regard, the Convention’s provisions on mutual legal assistance will be examined, both which are deemed essential to enhance States’ capabilities in relation to core international crimes cases.