A member of Buenaventura Durruti's insurgent group Los Solidarios, Sanz participated in the anarchist armed struggle against the Spanish monarchy and the dictatorship of Primo de Rivera.

Following the proclamation of the Second Spanish Republic, he became a leader of the Confederación Nacional del Trabajo (CNT) and Federación Anarquista Ibérica (FAI) in Catalonia.

In command of the Aragon front, he attempted multiple unsuccessful offensives against Zaragoza, but was either hampered by severe weather or held back by the Ministry of Defence.

[1] Formed by Buenaventura Durruti and Francisco Ascaso, the small group attacked trains and robbed banks associated with the Rio Tinto mining company, in actions that Sanz participated in.

[10] Seeking to avenge Durruti's death, on the night of 21 December 1936, Sanz led the column into an offensive against the Nationalist-held city of Zaragoza.

[12] Sanz continued to face difficulties on the Aragon front, as sectarian disputes between the anarchists and the Unified Socialist Party of Catalonia (PSUC) overtook meetings of the Republican command staff.

"[13] The intervention of the Ministry of Defence reached its height over the subsequent months, after it was decreed that the militias be militarised into the Spanish Republican Army, a decision that many anarchists fiercely opposed.

[15] With the end of the militia system in sight, in April 1937, Sanz ordered one last offensive against the cities of Zaragoza and Huesca, hoping a victory would delay their militarisation.

José Ignacio Mantecón [es], the newly-appointed military governor of Aragon, was disliked by Sanz, who called him a señorito (little lord).

[18] Having failed to achieve his objective of capturing Zaragoza, after the Nationalist victory in the Aragon Offensive, Sanz fled to France and was interned in a number of French concentration camps.