According to the International Committee of the Red Cross (ICRC), the duty to ‘ensure respect’ in Common Article 1 of the Geneva Conventions includes an external dimension: States have to ensure that other states respect their obligations under the Conventions, and arguably under international humanitarian law as a whole. This potentially has far-reaching consequences for states engaging in military operations together with partners, and the ICRC interpretation has been met with explicit pushback from a number of states as well as scholars. The present article seeks to revisit the debate on the extent of the obligation following from Common Article 1 in the context of partnered operations. We begin by providing a detailed outline of the ICRC view on the duty to ‘ensure respect by others’, as well as the reactions from states and scholarly contributions. Against this background, we examine—by carefully applying the customary rules on treaty interpretation enshrined in the Vienna Convention of the Law of Treaties—whether the duty to ensure respect has an external dimension judged by the ordinary meaning and intentions of the drafters, subsequent practice (both in relation to the ICRC and in other contexts) and judicial pronouncements. Having concluded that doctrinal legal analysis does indeed support an external dimension of Common Article 1, we then explore whether this duty to ensure respect by others applies to states regardless of the nature of the armed conflict and their own involvement in it. Finally, we analyse the exact content of the obligations arising from the duty to ensure respect by others under Common Article 1.