The First Mixed Brigade was established from the Fifth Regiment on 10 October 1936 in Alcalá de Henares as a result of the reorganization of the Spanish Republican Armed forces. It was put under the command of Communist commander Enrique Lister. The first combat action of the First Mixed Brigade was the Battle of Seseña. It would suffer many losses in the Battle of Brunete, including its Cuban brigade commander Alberto Sánchez, as well as a great number of officers and Chief of Staff Major Emilio Conejo. Later the First Mixed Brigade would see action in the Aragon Offensive, the Battle of the Ebro and the Catalonia Offensive.