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. One of these battalions became the "Fifth Regiment" ("5º Regimiento de Milicias Populares"), a military unit intended as a model for other military units to follow in the initial chaotic period of the civil war. Its first commander was Enrique Castro Delgado. Later the Fifth Regiment would take an active part in the battles of Somosierra and Guadarrama, as well as in the Battle of Talavera and the Siege of the Alcázar, ending up becoming one of the crucial military units engaged in the Defence of Madrid.