This article analyses situations in which States and international organizations partnering in military operations can bear responsibility in connection with conduct attributed to another. When engaging in military cooperation, a variety of international norms providing for obligations in relation to others should be taken into account. These include negative obligations not to assist or direct military partners in engaging in conduct violating international obligations, and positive obligations to take steps to ensure that military partners do not commit wrongful conduct. Taken together, they result in a framework which regulates military collaboration by determining thresholds where implication in the conduct of another, or lack thereof, engages responsibility. The aim of this article is to clarify and to conceptualise this framework, so as to provide an analytical background on the basis on which military officials can determine the proper balance between excessively permissive attitudes fostering violations and unnecessarily precautionary approaches hindering military cooperation. Based on a comprehensive review of relevant rules found in the ILC articles on the responsibility of States and of international organizations, international humanitarian law, and international human rights law, the article identifies four key criteria to allocate responsibility in connection with the wrongful conduct of military partners: knowledge, capacity, diligence, and proximity. In addition, the article offers some perspectives on the apportionment of legal consequences such as reparation.