In the face of the void thus created, the Communist Party of Spain led the implementation of a policy that sought to replace the spontaneous and disorganized bands fighting for the Spanish Republic with loyal, disciplined and militarized units.

[3] Finally, the Communist-led Antifascist Worker and Peasant Militias (MAOC) formed five battalions that took an active part in the Siege of Cuartel de la Montaña on 20 July 1936.

However, it soon attracted members from other ideologies owing to its efficiency and capacity for organization in a critical time for the Republic, as compared to the chaotic militia groups that operated at the onset of the war.

In the Fifth Regiment soldiers were not allowed to discuss the orders given by their superiors, a common practice in most of the spontaneous militia units that sprung up when weapons were issued to the trade unions.

[1] Besides Enrique Líster, other important leaders of the Spanish Republican Army such as Juan Guilloto León "Modesto", Valentín González "El Campesino" and Etelvino Vega Martínez, were formed in the ranks of the Fifth Regiment.