The United Nations Mission for the Referendum in Western Sahara (MINURSO) was established by Security Council resolution 690 of 29 April 1991. The settlement plan, as approved by the Security Council, provided for a transitional period for the reparation of a referendum in which the people of Western Sahara would choose between independence and integration with Morocco.
More Information: https://peacekeeping.un.org/en/mission/minurso