Following the adoption of UNSCR 1244, General Jackson, acting on the instructions of the North Atlantic Council, made immediate preparations for the rapid deployment of the security force (Operation Joint Guardian), mandated by the United Nations Security Council. The first NATO-led elements entered Kosovo at 5 a.m. on 12 June. On 21 June, the UCK undertaking of demilitarization and transformation was signed by COMKFOR and the Commander in Chief of the UCK (Mr. Hashim Thaci), moving KFOR into a new phase of enforcing the peace and supporting the implementation of a civil administration under the auspices of the United Nations.