The role of NGOs in influencing policymaking at both national and international levels has been widely studied. Yet, it is increasingly acknowledged that civil society organizations can also establish cooperative relationships with (international) military institutions to design and improve policies. We see this clearly in the design of Protection of Civilians policies, where non-governmental organizations (NGOs) such as PAX or the Center for Civilians in Conflict (CIVIC) participate in workshops with military staff. However, while the conditions under which NGOs and interest groups gain access to policy institutions are widely studied, little is known about the military sphere. Under what conditions do military institutions open up for cooperation with NGOs? We answer this question by studying a typical case of NGO-military cooperation towards policy improvement, which is the involvement of NGOs PAX and CIVIC in the development of the NATO Policy for the Protection of Civilians. We build on resource exchange theory, hypothesizing that military staff cooperate with NGOs when they perceive that their resources, e.g. expertise about on-the-ground repercussions of the use of force, can contribute to improving their practices and bring greater transparency about military operations. Empirically, we follow a dual-track approach, relying on interviews with NGO staff to gain insights in their supply of expertise, and with NATO military staff to explore their demand for expertise. The findings demonstrate that PAX and CIVIC seek to cooperate with NATO military staff to improve civilian protection practices at the intergovernmental level, with the aim that policy changes will eventually be transferred to the national military level. Moreover, NGO success in accessing military institutions such as NATO depends on the resources they can contribute. This study therefore adds to the literature on NGO strategies and IGO-NGO relations by showing that NGOs can access IGO policymaking even on “high politics” issues.