In November 1947 , the United Nations General Assembly endorsed a plan for the partition of Palestine , providing for the creation of an Arab State and a Jewish State, with Jerusalem to be placed under international status. The plan was not accepted by the Palestinian Arabs and Arab States. On 14 May 1948 , the United Kingdom relinquished its mandate over Palestine and the State of Israel was proclaimed. On the following day, the Palestinian Arabs, assisted by Arab States, opened hostilities against Israel .
On 29 May 1948 , the Security Council, in resolution 50 (1948) , called for a cessation of hostilities in Palestine and decided that the truce should be supervised by the UN Mediator, with the assistance of a group of military observers . The first group of military observers, which has become known as the United Nations Truce Supervision Organization (UNTSO), arrived in the region in June 1948. In 1949, UNTSO military observers remained to supervise the Armistice Agreements between Israel and its Arab neighbours, which were for many years the main basis of the uneasy truce in the whole area. UNTSO's activities have been and still are spread over territory within five States, and therefore it has relations with five host countries -- Egypt , Israel , Jordan , Lebanon and Syrian Arab Republic .
Following the wars of 1956, 1967 and 1973, the functions of the observers changed in the light of changing circumstances, but they remained in the area, acting as go-betweens for the hostile parties and as the means by which isolated incidents could be contained and prevented from escalating into major conflicts.
UNTSO personnel have also been available at short notice to form the nucleus of other peacekeeping operations. The availability of UNTSO's military observers for almost immediate deployment after the Security Council had acted to create a new operation has been an enormous contributory factor to the success of those operations.
In the Middle East , groups of UNTSO military observers are today attached to the peacekeeping forces in the area: the United Nations Disengagement Observer Force (UNDOF) in the Golan Heights and the United Nations Interim Force in Lebanon (UNIFIL). A group of observers remains in Sinai to maintain a United Nations presence in that peninsula. In addition, UNTSO maintains offices in Beirut and Damascus.
(Source: http://www.un.org/Depts/dpko/missions/untso/background.html )