Since joining the UN in 1955, its main bodies pressured Spain to proceed with the decolonization of "Spanish Sahara", which shortly after, and under the name of Western Sahara, was included in the list of Non-Self-Governing Territories. When Spain was preparing the referendum of self-determination in 1975, the General Assembly asked for it to be suspended until the International Court of Justice issued an advisory opinion. This opinion established that in 1884 there were no ties of sovereignty between Morocco and Mauritania and the people of the Territory and that the conflict should be resolved through the exercise of the right of self-determination. Nevertheless, following the signing of the Madrid Agreements and the withdrawal of Spain from the territory, Morocco and Mauritania occupied it, forcing the Saharawi people to start a war of national liberation that would last until 1991. That year launched a Peace Plan negotiated by Morocco and the POLISARIO Front, which provided for the holding of the referendum on self-determination.