The high number of casualties suffered by the XV International Brigade during the month of February 1937 prompted the General Staff of the Spanish Republican Army to create the 20th Battalion at the time of the Fascist Italian attack in Guadalajara in March.

The 20th Battalion was formed by remnants of the 21st Battalion garrisoned at Pozo Rubio, Albacete, new volunteers, as well as some soldiers and officers who had recovered from their wounds.

[1] It was moved to Puertollano—at the edge of the Sierra Morena—and put under the orders of the Chief of the Army of Andalusia on 20 March.

Eventually the 20th Battalion became the nucleus around which the 86th Mixed Brigade would be established around the first half of April 1937.

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