State responsibility for complicity in and other contributions to international crimes and serious human rights violations

International crimes and serious human rights violations are rarely committed by one actor, acting alone. The principal actor, be it a State or non-State actor, will often receive help and support from secondary actors, including States. This study questioned how international law can assign responsibility to States for complicity in or other contributions to international crimes and serious human rights violations, especially extraterritorially. If the conditions for complicity are not fulfilled, States may still be responsible for contributing to the commission, the continuation, or the risk of the principal violation on the basis of other legal rules. Which rules apply depends on the context, where the nature of the principal violation, the nature of the State’s assistance, and the victim’s physical location are particularly decisive. By bringing all these rules together, this study presents a clearer picture of which rules apply when States assist other actors in volatile situations, and how these rules interrelate. The rules selected for this study have been organised in four categories: complicity rules proper; rules prohibiting contributions below the level of complicity; rules prohibiting conduct in relation to a risk; and rules requiring States to regulate or investigate. Just because many, if not all forms of assistance qualify as breaches of an obligation to prevent, this does not mean that States should bear responsibility for failing to prevent the principal violation alone. States can also incur responsibility under human rights treaties when they are complicit in or otherwise contribute to serious human rights violations abroad.