Once again succession presented a problem in the Spanish monarchy. After three marriages and no heir, Ferdinand VII wed once again after becoming a widower. Until that moment his brother Carlos was next in line to the throne and he had big pretensions. A soap opera was beginning in the Spanish royal family and a new episode in war was underway: The First Carlist War.
In 1830 the new queen got pregnant, opening new possibilities for Ferdinand's direct descendants and 6 months before she gives birth, the king publishes a law rendering obsolete the Salic Law (which excluded women from having access to the throne) much to Don Carlos' chagrin.
Just as Ferdinand had foreseen might happen, his first born was a girl whom he named Isabella. The king died only three years later and little Isabella II, at the tender age of 3 was proclaimed queen with her mother Mara Cristina de Borbón y Dos Sicilias as regent.
Infant Carlos, Count of Molina had not recognized the child as the Princess of Asturias and hence refused to recognize her as the queen. Instead he was proclaimed king by the opposition, and thus the war begins.
The question of the succession was not only a dynastic matter, it was also political. A few years earlier the king had started flirting with some liberal ideals. During the "trienio liberal" the inquisition had been abolished and Ferdinand had not restored it. Carlos pretended, as king, to return to the utmost absolutism, in which the church and the state would be one, among other thing. A big point for Carlos was his promise that the autonomous regions would keep their privileges, the First Carlist war begins.
Isabella II gathered support in the high classes of the Spanish army, the church and the state, as well as the liberals whom the regent Mª Cristina made a pact with as the only way to save the throne for her daughter. On the opposite corner, the Carlist force was made up of all who opposed the liberal revolution: small rural nobility, lower clerics, and many farmers that believed all that liberalism would bring was higher taxes.
The war began with the uprising of Carlists supporters in the Basque Country and Navarre and an important part of the clergy of Aragon and Catalonia joined them, although other cities in the north remained loyal to Isabella II, such as Pamplona, San Sebastian and Bilbao. The Carlists force acted as a guerrilla at first, until general Zumalacárregui is appointed to organize a Basque-Navarre army while general Cabrera united the Aragonese and Catalan fronts.
Even though it was an entirely Spanish war both sides had international support. The Carlists received money and weapons from absolutist states such as Russia, Prussia and Austria and the Portuguese who were loyal to Miguel I of Portugal. On the other hand Isabella II received the support and troops of England (British Auxiliary Legions, 10,000 men), France (French Foreign Legion, 5,000 men) and the Portuguese who were loyal to Pedro I of Brazil.
The war was waged over the next seven years with more than 120,000 soldiers dead and countless civilian victims, as an extended guerrilla and civil war it was far more brutal than other type of warfare as any prisoners caught would be executed right away. Zumalacárregui died in 1835 in the Bilbao siege. By 1836 the Carlist movement was divided between those that wanted to seek a peace agreement with the liberals commanded by Maroto and those who wished to continue the war.
In 1839 Maroto and Espartero, in charge of the liberal faction, signed a peace agreement, the Convention of Vergara. They agreed that the Basque provinces and Navarre would keep their privileges and that the Carlist soldiers would join the liberal forces. However the intransigent troops continued to fight a guerrilla until their complete defeat in 1840. Don Carlos had no choice but run away to France, which ended the First Carlist war .
Overall, the war helped the Spanish learn that their country was not in good shape.