The complex environments in which peace operations have to function often pose difficult internal security challenges, such as the demilitarization of nonstate militias, control and seizure of heavy and light weapons, protection of humanitarian aid/zones, deterrence of anarchy and crime in situations of state collapse, prevention of interethnic violence, and control of porous and/or contested borders. Security sector reforms have been central to the mandates of Balkans peace operations, but difficulties in responding to specific security challenges have often led to operational setbacks in the field. Effective management of such challenges has thus been an increasingly important priority for both the United Nations and European security organizations, such as the North Atlantic Treaty Organization (NATO) and the Organization for Security and Cooperation in Europe (OSCE). But while NATO can provide the requisite forces for a strong military deterrent, it cannot contribute the political, administrative, legal, and economic elements necessary for effective security sector reform. Joint action by both military and civil actors is necessary for successful management of the security sector. And successful security sector management is in turn central to the broader strategic goal of fostering a sustainable peace-building dynamic. (1) In this article, I examine the recent experience of civil-military responses to security sector issues in the United Nations Interim Administration Mission in Kosovo (UNMIK) and the NATO-led Kosovo Force (KFOR). It should be noted here that UNMIK and KFOR have been the most successful complex Balkans operations to date for interorganizational interaction and have demonstrated that many post-Bosnia lessons were effectively applied in the international peacebuilding response to the 1999 Kosovo conflict. A comparison of evolving Balkans security sector action, from Bosnia and Eastern Slavonia to Kosovo, suggests that the civil-military interface should move from cooperative relations to coordinated unity of effort. Nevertheless, it is important to stress that Kosovo is not in a truly condition, as previous Balkans theaters have been. The key elements for postconflict status--a peace agreement and/or determination of a final end state for the territory--are glaringly absent in Kosovo and have been from the arrival of international actors in the contested province. Indeed, Kosovo's indeterminate political status is at the root of many of its most intractable threats to public security, including endemic violence against ethnic minorities, particularly Serbs, and the consequent emergence of polarized ethnic enclaves. Civil-Military Interaction and the Imperative for Coordination The inherent complexity of Balkans conflict environments has demanded interaction between NATO's military forces and the civilian organizations, such as the UN and the OSCE, also active in addressing security sector issues. The capacity of the UN is challenged where deterrent military force is needed in a rapid and massive manner. This need has been the norm for the Balkans, with Bosnia and Kosovo both requiring tens of thousands of well-equipped forces able to project a credible war-fighting face. The fact that NATO is the only organization in the region with the capacity to mount such missions was recognized early on by Kofi Annan. In 1993, he advocated UN-NATO cooperation in the delivery of missions that needed peacekeeping with teeth, noting that their size and complexity make it imperative to explore new avenues of cooperation with regional organizations such as NATO. (2) The interrelated nature of security challenges in these missions has also meant that what used to be understood as a clear distinction between military and civil roles has become increasingly indistinct. While NATO has taken some concrete steps to institutionalize aspects of civil-military interaction in the field, these have generally been only to the extent of formalized cooperation. …