State responsibility for conduct of private military companies violating ius ad bellum

This chapter discusses the circumstances under which states can be held accountable for a breach of the ius ad bellum, when the acts in question were performed by a private military and security company (PMSC). The definition of the ius ad bellum itself is reconsidered as a corpus of norms, including a prohibition of the use of force but also encompassing obligations to prevent this use of force by private actors such as PMSCs. This lato sensu definition may lead to a recognition of state responsibility irrespective of whether or not the acts of private contractors are attributable to the state. It is argued that when a state cannot be held accountable for the violation of ius ad bellum stricto sensu by a PMSC, as a next step in determining state responsibility, respect of the due diligence principle needs to be examined. Moreover, states may have a duty to harmonise their national legal systems in order to prevent ius ad bellum violations by private actors