The Republican general staff reportedly concluded that they should turn this makeshift model into a systematic and structured scheme and units raised in this way should emphasize mobility and flexibility rather than manpower.

Initially the reason were pressing wartime necessities; the Nationalist army was approaching Madrid and half-formed units were thrown into battle before they reached the structure envisioned.

[20] Later it turned out that the Republic was unable to provide all weapons and equipment needed, and that shortage of NCOs and officers made it impossible to ensure their sufficient number even for traditional army units.

Finally, in later stages of the war the increasingly dramatic military situation forced Republican high command to raise troops in non-systematic, makeshift manner.

[23] The result was decreasing firepower and increasing manpower of the entire unit; final versions of the brigada mixta scheme envisioned its strength as some 4,200 people, though less than a half would serve in frontline combat sub-units.

Manpower was provided by distributing pre-war regiments among the brigades,[34] by incorporating earlier militia units active on the front, and then by regular draft (though there were also volunteers).

[38] Non-infantry sub-units were increasingly smaller; as general staff experts noted artillery was too dispersed, to gain critical mass guns and howitzers were shifted to divisional units.

[39] Deployment of mixed brigades differed significantly from the original plan; instead of operating as autonomous units, they were grouped by 3 into divisions[40] and remained their fixed components.

[43] Scholars note that the underlying assumptions which gave rise to the mixed brigade concept were basically correct, and that the nature of warfare during the Spanish Civil War indeed favored mobility, flexibility and autonomy over structure, manpower and hierarchical organization.

They also note that the innovative idea of creating combined-arms units below the divisional level was later successfully implemented during World War Two in form of the improvised German Kampfgruppen.

[45] Another point is that the Republic was unable to provide sufficient equipment and arms, and as a result non-infantry sub-units remained to a large extent an under-armed and under-equipped fiction.

[47] One military historian claims that "thanks to the decision to adopt the mixed brigade as its basic unit, the People's Army was crippled as a fighting force almost from its very inception".