José Asensio was born in Spain in 1892. He joined the Spanish Army and had reached the rank of colonel by the age of thirty-four.

On the outbreak of the Spanish Civil War Asensio joined the armed forces supporting the Popular Front government. He was appointed as chief of staff to General José Miaja who was given responsibility for defending Madrid.

When Francisco Largo Caballero became prime minister he immediately promoted Asensio to general and appointed him to command the central theatre of operations. Asensio was unpopular with the Anarcho-Syndicalists and Communists and after the failure at Illescas he lost his post.

Largo Caballero continued to have faith in Asensio and appointed him undersecretary of war in September, 1936. In this role he helped develop the new Republican Army. Asensio insisted on proper uniforms and the traditional hierarchy. He decided that Militias should be placed under military law and schools were established to train future officers in the army.

Asensio was blamed for the loss of Málaga in February 1937 and was dismissed from office. In September he was arrested and imprisoned. He was accused of incompetence but he was released in 1938 without charge.

In January 1939 Asensio was appointed military attaché to the Spanish Embassy in Washington. He remained in the United States after the war.