If there is one historical event that people associate with the beret, it is most likely the Spanish Civil War (1936-39).
However, the three Carlist wars between 1833 and 1876, formed the beginning of the use of the beret as a piece of uniform and identification for a political group.
In the beginning the ultra conservative catholic Carlists wore the traditional black beret that was already worn in the Basque region. After the Battle of Alsasua (22 April, 1834) the captured Liberal troops from La Mancha, Valencia and Andalusia that chose to join the victors (and that way stay alive) were given distinctive (French made) red berets which were rejected by the regular Carlist troops.
Originally the red berets were only worn by this 'elite' battalion of Guías de Navarra (Navarese Guides). The regular Carlist troops chose distinctive blue berets. The red beret soon gained popularity among all Carlists, despite turning the wearers into great target points for sharpshooters. Since then the red beret (with or without tassels) is very much their symbol.
I quote from J. MacClancy's 'The Decline of Carlism':
"The beret became to signify Carlism and all it implied."
"One requité, about to be shot by a Republican firing squad who have removed his beret, shouts: Please, leave me the beret! I want to die with it on as the best safe conduct to present myself before the Tribunal of God, for whom I die."
In the Spanish Civil War of 1936-39 Carlists still wore their red beret, with tassel. Their opponents – Republican, communist and anarchist militias and the International Brigaders also wore berets. Theirs were generally a black or navy blue beret (traditional head gear of the Spanish workers) or a khaki military style beret.
Ernest Hemingway donned a beret during the conflict, and for many International Brigaders it was a first priority to acquire a beret upon arrival in