He served in the Army of the Spanish Republic from July 18, 1936 until the end of war. In 1936 he was the chief of staff of Kleber. Later, he joined the PCE. After that, he was the republican commander of one Mixed Brigade in the Second Battle of the Corunna Road in November 1936 and in the Segovia Offensive and the Battle of Brunete he led the 69th division. He covered the retreat of the republican forces in the Maestrazgo during the Aragon Offensive and was one of the Republican commanders in the defense of the XYZ Line in 1938. He also served briefly in "SIM", the ("Servicio de Investigación Militar"), as chief of the department for the Army of the Centre. In March 1939, when Franco's troops had reached Valencia, Durán escaped from Gandia, Spain, aboard a British destroyer, landing at Marseille and, eventually, London.