With the onset of World War II, Bolivia’s tin supply became increasingly important to the U.S. war effort. Mineworkers were also increasingly aware of the value of their labor and began to organize and pressure for increases in their modest wages. The mine-owners likewise pressured the government of President General Enrique Peñaranda (1940–43) to suppress trade-union and strike activity. The conflict came to a head in Simón Patiño’s Catavi mine on 21 December 1942, when troops opened fire on a demonstration of workers. The government claimed that nineteen protestors died and thirty were wounded, while the opposition cited hundreds of casualties.

The massacre generated international repudiation, including in the United States, where the social-democratic Franklin D. Roosevelt administration found itself in the awkward position of importing tin stained with the blood of Bolivian workers. Within Bolivia, the incident provoked dramatic parliamentary debate in 1943, as the recently formed Revolutionary Nationalist Movement (mnr) and the Marxist Revolutionary Left Party (pir) denounced the nexus between a repressive government, mineowning plutocrats, and United States imperialism. The most compelling voice heard was that of a young mnr congressman from Tarija, Víctor Paz Estenssoro. Astutely, Paz Estenssoro sought to rehabilitate the image of the armed forces while condemning the government, and to distance the mnr from the Marxist left while seeking to represent proletarian workers.

The massacre catalyzed major political changes in the country. The powerful movement within Congress to censure the massacre dealt a severe blow to the Enrique Peñaranda government, which, in late 1943, was overthrown in a coup that joined the ascendant mnr with dissident military offcers led by General Gualberto Villarroel. In 1944, under the Villarroel government, the miners formed their first national labor organization, the Trade Union Federation of Bolivian Mineworkers (fstmb). Its identity would be forever marked by the memory of the martyrs of Catavi.