Transitions in the aftermath of international military interventions are precarious. Often, they have seen the onset or re-emergence of protracted periods of violence and instability. This thesis investigates how the United Nations (UN) has planned for these complex operational environments. In so doing, it examines how the UN’s legitimation attempts – that is, the intentional efforts on the part of UN officials to bring about, enhance, or reassert consistency between the UN’s behaviour and principles that are fundamental to its institutional identity – have shaped the planning and design of its post-intervention field operations. With a view to these research objectives, the theoretical and empirical analysis for this thesis precedes in three principal steps. First, the thesis draws on existing literature to theorise that in the aftermath of military interventions the UN’s legitimacy revolves around its compliance with two key institutional principles: the need to respect a local population’s right to selfdetermination and the imperative to operate with impartiality. In drawing on a qualitative case study analysis of the UN’s pre-mission planning efforts for two post-intervention field operations – the United Nations Transitional Administration in East Timor (UNTAET) and the United Nations Support Mission in Libya (UNSMIL) – the thesis demonstrates that UN officials were pre-eminently concerned with the UN’s compliance with these key organisational principles in these contexts of post-intervention transition. In building on this finding, the study investigates how the UN’s legitimation attempts shaped its planning and design of post-intervention field operations. In so doing, the thesis shows that when concerns for the UN’s compliance with the principle of impartiality took precedence, UN officials adopted policies that centralised governing authority under the auspices of the UN mission, while delaying the transfer of authority to local actors. By contrast, when concerns for the UN’s compliance with the principle of self-determination were preeminent, this predicated policy decisions that privileged the recognition of local actors and the transfer of leadership and authority to local interlocutors. On this basis, the study finds that the relative priority given to each of these legitimacy concerns had a tangible impact for the scope and nature of the UN’s post-intervention engagements by informing differing policy approaches and priorities. However, this study also finds that even when one of these legitimating principles was clearly prioritised in policy deliberations, UN officials engaged in concerted efforts to (re)interpret and (re)cast the other principle in such a way as to allow the UN to claim compliance with both key standards of institutional legitimacy.