NATO first became involved in the Bosnian conflict in support of the United Nations to enforce economic sanctions, an arms embargo and a no-fly zone, and to provide military contingency planning. These measures helped moderate the conflict and save lives, but proved inadequate to end the fighting. By contrast, NATO's 12-day air campaign in September 1995 paved the way for the Dayton Agreement, the peace accord ending the Bosnian conflict that came into force on 20 December 1995.
Under the terms of the peace agreement, NATO deployed peacekeepers for the first time, leading a 60,000-strong Implementation Force (IFOR). The deployment of IFOR, which included soldiers from both NATO and non-NATO countries, was the Alliance's first major operational military engagement on land and has contributed greatly to reshaping its post-Cold War identity. Under a UN mandate, IFOR's mission was to secure an end to hostilities; to separate the armed forces of the war-torn country's newly created entities, the Federation of Bosnia and Herzegovina and Republika Srpska, and to place their heavy weapons in secure cantonments; and to transfer territory between the two entities.
Within a year, IFOR completed its work and was replaced by a smaller Stabilisation Force (SFOR) in December 1996. In addition to deterring a resumption of hostilities and promoting a climate in which the peace process could move forward, SFOR's mission was extended to include support for civilian agencies involved in the international community's efforts to build a lasting peace in the country. The peacekeepers helped refugees return home, provided security for elections, and contributed to reforming the Bosnian military forces. They also assisted in the apprehension of indicted war criminals.
As the security situation improved, the number of SFOR troops was reduced progressively. Eventually, in December 2004, NATO handed responsibility for day-to-day security in Bosnia and Herzegovina to the European Union. However, the Alliance retains a military headquarters in Sarajevo, which focuses mainly on defence reform and preparing the country to join NATO's Partnership for Peace programme.