The Western Sahara War was an armed conflict between the Sahrawi Indigenous Polisario Front and Morocco from 1975 to 1991 (and Mauritania from 1975 to 1979), being the most significant phase of the Western Sahara conflict. The conflict erupted after the withdrawal of Spain from the Spanish Sahara in accordance with the Madrid Accords (signed under the pressure of the Green March), by which it transferred administrative control of the territory to Morocco and Mauritania, but not sovereignty. In late 1975, the Moroccan government organized the Green March of some 350,000 Moroccan citizens, escorted by around 20,000 troops, who entered Western Sahara, trying to establish a Moroccan presence. While at first met with just minor resistance by the Polisario Front, Morocco later engaged a long period of guerrilla warfare with the Sahrawi nationalists. During the late 1970s, the Polisario Front, desiring to establish an independent state in the territory, attempted to fight both Mauritania and Morocco. In 1979, Mauritania withdrew from the conflict after signing a peace treaty with the Polisario Front. The war continued in low intensity throughout the 1980s, though Morocco made several attempts to take the upper hand in 1989–1991. A cease-fire agreement was finally reached between the Polisario Front and Morocco in September 1991. Some sources put the final death toll between 10,000 and 20,000 people.

The Western Sahara conflict has since shifted from military to civilian resistance. A peace process, attempting to resolve the conflict has not yet produced any permanent solution to Sahrawi refugees and territorial agreement between Morocco and the Sahrawi Republic. Today most of the territory of Western Sahara is under Moroccan occupation, while the inland parts are governed by the Sahrawi Arab Democratic Republic, managed by the Polisario Front.

In 1884 Spain claimed a protectorate over the coast from Cape Bojador to Cap Blanc. Later, the Spanish extended their area of control. In 1958 Spain joined the previously separate districts of Saguia el-Hamra (in the north) and Río de Oro (in the south) to form the province of Spanish Sahara.

Raids and rebellions by the indigenous Saharan population kept the Spanish forces out of much of the territory for a long time. Ma al-Aynayn, the Saharan Caïd of Smara, started an uprising against France in the 1910s, at a time when the French had expanded their influence and control in North-West Africa, he died in the same year and his son Ahmed al-Hiba succeeded him. French forces defeated him when he tried to conquer Marrakesh, and in retaliation, severely damaged the holy city of Smara in 1913. Not until the second destruction of Smara in 1934, by joint Spanish and French forces, did the territory finally become subdued. Another uprising in 1956–1958, initiated by the Moroccan Army of Liberation, led to heavy fighting, but eventually the Spanish forces regained control, again with French aid. However, unrest simmered, and in 1967 the Harakat Tahrir arose to challenge Spanish rule peacefully. After the events of the Zemla Intifada in 1970, when Spanish police destroyed the organization and "disappeared" its founder, Muhammad Bassiri, Sahrawi nationalism again took a militant turn.

In 1971 a group of young Sahrawi students began organizing what came to be known as The Embryonic Movement for the Liberation of Saguia el-Hamra and Rio de Oro. After attempting in vain to gain backing from several Arab governments, including both Algeria and Morocco, but only drawing faint notices of support from Libya and Mauritania, the movement eventually relocated to Spanish-controlled Western Sahara to start an armed rebellion.

The Polisario Front was formally constituted on 10 May 1973 in the Mauritanian city of Zouérat, with the express intention of militarily forcing an end to Spanish colonization. Its first Secretary General was El-Ouali Mustapha Sayed. On 20 May he led the Khanga raid, Polisario's first armed action, in which a Spanish post manned by a team of Tropas Nomadas (Sahrawi-staffed auxiliary forces) was overrun and rifles seized. Polisario then gradually gained control over large swaths of desert countryside, and its power grew from early 1975 when the Tropas Nomadas began deserting to the Polisario, bringing weapons and training with them. At this point, Polisario's manpower included perhaps 800 men, but they were backed by a larger network of supporters. A UN visiting mission headed by Simeon Aké that was conducted in June 1975 concluded that Sahrawi support for independence (as opposed to Spanish rule or integration with a neighbouring country) amounted to an "overwhelming consensus" and that the Polisario Front was by far the most powerful political force in the country.^{[citation needed]}

While Spain started negotiating a handover of power in mid 1975, it ceded the administrative control of the territory to Mauritania and Morocco only after signing the Madrid Accords. However, on 31 October 1975, Moroccan troops crossed into the territory from the north-east, advancing towards Mahbes and Farciya.

The Moroccan government organized the Green March of some 350,000 Moroccan citizens, escorted by around 20,000 troops, who entered Western Sahara, trying to establish Moroccan presence. While at first meeting just minor resistance by the Polisario, Morocco had later engaged in a long guerrilla warfare with the Sahrawi nationalists. During the late 1970s, after Moroccan pressure through the Green March of 6 November, Spain entered negotiations that led to the signing of the Madrid Accords by which it ceded unilaterally the administrative control of the territory to Mauritania and Morocco on 14 November 1975. The United Nations did not officially recognize the accord, considering Spain as the administrative power of the territory. In late 1975, as a result of the Moroccan advance, tens of thousands of Sahrawis fled Morocco-controlled cities into the desert, building up improvised refugee camps in Amgala, Tifariti and Umm Dreiga.