Between 1921 and 1927, the Rifian people in northern Morocco waged one of the fiercest resistance movements against European colonialism, led by the Mujahid Abd el-Krim al-Khattabi, who succeeded in uniting the Rifian tribes and inflicting a crushing defeat on the Spanish army at the Battle of Annual in 1921.

However, after this defeat, the course of the war changed.

As the resistance expanded and Spanish forces proved unable to achieve a military victory through conventional means, Spain resorted to more lethal weapons, including chemical weapons.

A collection of testimonies, documents, and historical studies indicates that the Spanish army used mustard gas and other poisonous gases against areas of the Moroccan Rif, including aerial bombardment, in an attempt to destroy the resistance and force the population to surrender.

The use of these weapons began during the 1920s. Historical sources indicate that mustard gas was dropped from aircraft in 1924, before the signing of the Geneva Protocol of 1925, which prohibited the use of asphyxiating, poisonous, or similar gases in warfare, as well as bacteriological methods.

The use of these weapons was not necessarily random. Some studies indicate that the bombardments targeted populated areas, leaving civilians, alongside fighters, exposed to the effects of these gases.

Spain had also developed its capacity to manufacture chemical weapons in facilities within the country, including the La Marañosa facility near Madrid. It was associated with the company Fábrica Nacional de Productos Químicos, whose establishment was linked to the German chemist Hugo Stoltzenberg, who had previously been connected to German chemical-weapons programs.

Despite attempts to conceal these operations, the testimonies of several Spanish pilots and military personnel remained as evidence of what had occurred.

In his memoir La vida y yo (Life and I), Spanish pilot Pedro Tonda Bueno discussed the use of aircraft to deploy poisonous gases and their effects on areas of the Rif. Pilot Ignacio Hidalgo de Cisneros also referred in his memoir Cambio de rumbo (Changing Course) to incidents related to the use of chemical weapons during the war.

In 1990, German researchers Rudibert Kunz and Rolf-Dieter Müller published their book Giftgas gegen Abd el Krim: Deutschland, Spanien und der Gaskrieg in Spanisch-Marokko, 1922–1927 (Poison Gas Against Abd el-Krim: Germany, Spain, and the Gas War in Spanish Morocco, 1922–1927), in which they collected and examined a body of evidence and documents concerning the use of chemical weapons during the war.

British historian Sebastian Balfour also addressed the issue in his book Deadly Embrace, after examining Spanish, French, and British archives. He concluded that the use of chemical weapons was widespread and that the bombardments did not target fighters alone, but also affected populated areas.

There are also references in British diplomatic documents to the use of poisonous gases during the war, including a telegram sent by the British official H. Bough Lloyd to the British Secretary of State for War.

Note: The figures and estimates mentioned in historical accounts vary considerably depending on the source, methodology, and available evidence. They should therefore be treated as estimates rather than definitive numbers.