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 an uneasy truce in the area. UNTSO activities are spread over the five States comprising the parties to the Armistice Agreements: Egypt, Israel, Jordan, Lebanon and the 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 on a temporary basis. The ability of UNTSO military observers to deploy after the Security Council acted to establish a new operation has been an important factor contributing to the initial effectiveness of such missions.
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) on the Golan and the United Nations Interim Force in Lebanon (UNIFIL). In addition, UNTSO maintains liaison offices in Egypt, Lebanon, Syria, and Jerusalem for liaison with Israel and Jordan.
(Source: http://www.un.org/Depts/dpko/missions/untso/background.html )