ABSTRACTThe literature on peacekeeping has paid scant attention to the interaction between peacekeeping troops and host country military. Addressing this gap in scholarly knowledge, this paper conceptualizes such interaction as ‘diagonal interoperability’. The latter is situated in-between ‘horizontal interoperability’ on the one hand, relating to interaction between different components of a peacekeeping mission, and ‘vertical interoperability’ on the other, referring to the relations between international peacekeepers and ‘peace-kept’ populations. The paper focuses on the Democratic Republic of the Congo, where UN forces and the Congolese army are engaged in joint military operations and army reform is part of the peacekeeping mission’s mandate. Studying both mutual representations and joint practices, the paper explores the organizational, political, discursive, and security-related factors that shape diagonal interoperability. It concludes that diagonal interoperability between the two forces is weak, as reflected in mutual distrust and ‘not-so joint’ joint operations. Perhaps surprisingly, it finds that shared military identities do not seem to facilitate collaboration. Rather, mutual perceptions of the ‘military Other’ are infused with discourses of cultural and political difference, therefore accentuating the power asymmetries that undermine diagonal interoperability.