[8] At the beginning of the Spanish Civil War, Sanz volunteered for the Nationalists in opposition to the socialist government of Francisco Largo Caballero.

Sanz served as a truck driver in the Cuerpo de Ejército Marroquí, a unit of Francisco Franco's army created in 1937 and commanded by General Juan Yagüe.

Unknowingly, Sanz was part of the same side of the Italian Giorgio Perlasca, who would be his main partner during World War II in Budapest.

Sanz Briz dutifully informed the Spanish Foreign Ministry of his actions, that were deliberately permitted by Madrid through administrative silence, a typical diplomatic procedure used to not compromise the chancellery.

[10] Between June and December 1944, according to Giorgio Perlasca,[12] he and his assistants issued fake Spanish papers to 5,200 Jews, saving them from deportation to concentration camps.

[16] Sanz Briz himself tells how he was able to save the lives of so many Jews, in Federico Ysart's book Los judíos en España (1973).

He is also the subject of the 2011 Spanish television series El ángel de Budapest, based on Diego Carcedo's book Un español frente al Holocausto ("A Spaniard against the Holocaust").

In 2023, Committee on Culture and Sports [es] in the Spanish Senate rejected a proposal by far-right party Vox to commemorate Sanz Briz.