MINURSO was established by the UN in 1991 to monitor a ceasefire and prepare a referendum on self-determination that was never held due to voter eligibility disputes

From the articleRussia, US veto rival UN Western Sahara resolutions, leaving peacekeeping mission in limboAlso in Rival US and Russia resolutions on disputed Western Sahara are defeated in UN Security Council, Competing US, Russia resolutions on Western Sahara defeated in UN Security Council, Rival US and Russia resolutions on disputed Western Sahara are defeated in UN Security Council, UN Peace Mission to Western Sahara Hits Unprecedented Standoff, Rival US and Russia resolutions on disputed Western Sahara are defeated in UN Security Council