Thinking of Spain as a modern nation state today distorts the complicated reality that the Iberian Peninsula faced in the past. Spain was a nation in progress, consisting of regions united under the Spanish crown, but with strong regional identities based on different historical and cultural experiences. The largest entities were the kingdoms of León and Castile, but Spain also included the kingdoms of Navarre, Andalusia, Granada, Jaén, Aragon, and Valencia. There were also the principalities of Asturias and Catalonia, the lordship of Vizcaya, and both Guipúzcoa and Alava were "exempted provinces." Navarre, Aragon, and Catalonia had separate Cortes, which were versions of parliaments (Parker 18-19). This complex system of entities granted privilege to local power structures over the concept of a unified nation and made administration difficult, because there were few standards that applied to all of Spain. Many of the regions had special laws that respected and allowed traditional institutions, administrative patterns, and cultural patterns. These local and regional rights were called fueros and were fiercely defended against centralization. The fueros originated as rights agreed to when the regions joined the Spanish crown. Before becoming king, the king-designate had to swear to maintain and respect the fueros. This meant that the rights of the king were to a considerable extent limited.

Inevitably, liberals and centralizing monarchs alike tried to change the situation over the years, which produced political tensions. The Carlists promised to maintain the older system, which is why they were so firmly backed in the Basque regions by most of the peasants and nobility. The Carlist claimants were strong Catholics and were strongly supported by the Church, and thus by the more fervently Catholic portions of the population.