He was the chief of Azaña's military staff and worked as general co-ordinator to the minister of war.

In August 1936 he was appointed minister of war,[1] but was dismissed following the fall of Talavera in September.

[4] After Catalonia became an isolated enclave following the rebel Aragon Offensive in the spring of 1938, Hernández Saravia commanded the Eastern Region Army Group (Grupo de Ejércitos de la Región Oriental) and led the Republican Army during the overwhelming Francoist Catalonia campaign.

[5] Following the Fall of Barcelona on 27 January, Juan Hernández Saravia was relieved of his duties as leader of the Eastern Region Army Group, allegedly for his "defeatism" (derrotismo), but the real motives went far deeper.

[6] Hernández Saravia had tried to replace Juan Modesto, the head of the Ebro Army, whom he saw as self-important and not up to the mark[7] and hand over the command to Juan Perea, the more competent commander of the Eastern Army, but Modesto was protected by the highest leaders of the doomed Spanish Republic.