http://upload.wikimedia.org/wikipedia/commons/1/16/Cuadro_-Calderote-_Primera_Guerra_Carlistas_by_Ferrer_Dalmau.jpgThe Civil War of 1820-23 had restored power to the royalist faction, but still the liberals plotted. The fourth wife of Ferdinand VII was Maria Cristina, a princes of Sicily, and she was sympathetic to some Liberal reforms,—particularly those which sought to raise more money for a centralized government. She used her influence to convince Ferdinand to disregard the Salic laws, and leave the kingdom to his 3-year-old daughter, Isabella, rather than his brother Don Carlos, who had long expected to inherit the throne. This upset the balance of powers terribly, and before Ferdinand was cold in his grave—and long before Don Carlos began to take an active role in the rebellion, the royalists had raised militias, and set about to oppose the new government.
The army was strongly behind the Maria Cristina, regent for her daughter Isabella, and the Liberal leader Espartero was a capable general. The early battles went in favor of the Cristinos, as the Liberals called themselves. But the wealthy and powerful Basque region in Northern Spain was strongly royalist, and financed the raising of a substantial army to press the royalist cause. Taxing the prosperous trading regions of the north (who were largely self-governing and exempt from national taxes), confiscating Church property, and centralizing government power were the central aims of the liberals, and all were strongly resisted in the north and the rural districts.
The war was drawn out and particularly vicious. Both sides executed, and sometimes tortured their prisoners. In the cities, monks and religious were taunted and murdered, while in the towns captured by the royalists, liberal adherents abused. The first battles of the war were won by the Cristinos since they had a standing army at-the-ready. The liberals therefore controlled all of the major cities and the institutions of government for the duration of the war. But among the Carlists there arose a terrific general, Tomas Zumalacarregui and he won a series of major victories over the Cristinos from the time he first took up arms in 1833 until his death in 1835. He was particularly successful using a combination of both conventional and guerilla tactics against the Cristinos, and by April of 1845 the Carlists controlled all of the Peninsula north of the Ebro.
The death of Zumalacarregui deprived the Carlists of an effective leader, since Don Carlos, like all the Bourbon princes, was feeble and craven. Even so, the Carlists were not defeated for over four years, and probably would not have been, but for the military and financial resources of the British, who intervened on behalf of the liberals. A treaty to end hostilities was signed in 1839 by Espartero, who had replaced Maria Cristina as the regent for Isabella, and Maroto, a Carlist general. Although most of the Carlist army laid down arms after the treaty was signed, Maroto was considered a traitor by large segments of the population.