The Ifni War, sometimes called the Forgotten War (la Guerra Olvidada) in Spain, was a series of armed incursions into Spanish West Africa by Moroccan insurgents that began in November 1957 and culminated with the abortive siege of Sidi Ifni.

The city of Sidi Ifni had been ceded to the Spanish Empire in 1860 at the end of the Hispano-Moroccan War. After Morocco achieved independence in 1956, it sought to claim Spain's remaining possessions in West Africa. Violent demonstrations against Spanish rule broke out in Ifni in April 1957, and in October Moroccan militias began converging near the territory. Moroccan forces attacked in November, forcing the Spanish to abandon most of the territory and retreat to a defensive perimeter around Ifni. Supplied by the Spanish Navy from the sea, the Spanish garrison was able to resist the siege, which lasted into June 1958. In Spanish Sahara, Moroccan units, now reorganised as the Moroccan Army of Liberation, engaged in heavy fighting with Spanish forces at El Aaiún and Edchera. By February 1958, a joint Spanish and French offensive had driven the Moroccans out of Spanish Sahara.

Hostilities ceased in April 1958 (although small skirmishes still occurred) with the Treaty of Angra de Cintra, signed by the Spanish and Moroccan governments, by which Cape Juby and most of the Ifni territory were transferred to Morocco. The city of Sidi Ifni remained in Spanish possession until 1969, when, under international pressure, it was relinquished to Morocco.

Morocco ceded the city of Sidi Ifni to the Spanish Empire at the conclusion of the Hispano–Moroccan War in 1860 under the Treaty of Wad Ras, but Spain did not occupy the city until 1934. The following decades of Franco-Spanish collaboration resulted in the establishment and extension of the Spanish protectorate in Morocco south of the city to Cape Juby. In 1946, Spain's coastal and inland colonies were consolidated as Spanish West Africa.

When Morocco regained independence from France and Spain in 1956, the country expressed a keen interest in all of Spain’s remaining colonial possessions in Morocco, claiming that they were historically and geographically all part of Moroccan territory. Sultan Mohammed V encouraged efforts to re-capture the land and personally funded anti-Spanish guerillas to claim Ifni back for Morocco.

Violent demonstrations against Spanish rule erupted in Ifni on 10 April 1957, followed by civil strife and widespread killings of those loyal to Spain. In response, Spanish dictator Francisco Franco dispatched two battalions of the Spanish Legion, Spain's elite fighting force, to El Aaiún in Saguia el-Hamra in June.

Spanish military mobilisation resulted in the Royal Moroccan Army converging near Ifni. On 23 November 1957, two villages on the outskirts of Sidi Ifni, Goulimine and Bouizakarne, were occupied by 1,500 Moroccan soldiers (Moujahidine).

The encirclement of Ifni was the beginning of the Ifni War. Two more Legion battalions reached Spanish Sahara before the opening of hostilities.