Western Sahara is a disputed territory on the northwest coast of Africa bordered by Morocco, Mauritania, and Algeria. After Spain withdrew from its former colony of Spanish Sahara in 1976, Morocco annexed the northern two-thirds of Western Sahara and claimed the rest of the territory in 1979, following Mauritania's withdrawal. A guerrilla war with the Polisario Front contesting Morocco's sovereignty ended in a 1991 cease-fire and the establishment of a UN peacekeeping operation. As part of this effort, the UN sought to offer a choice to the peoples of Western Sahara between independence (favored by the Polisario Front) or integration into Morocco. A proposed referendum never took place due to lack of agreement on voter eligibility. The 2,700 km- (1,700 mi-) long defensive sand berm, built by the Moroccans from 1980 to 1987 and running the length of the territory, continues to separate the opposing forces with Morocco controlling the roughly 80 percent of the territory west of the berm. Local demonstrations criticizing the Moroccan authorities occur regularly, and there are periodic ethnic tensions between the native Sahrawi population and Moroccan immigrants. Morocco maintains a heavy security presence in the territory.