The Quadruple Alliance was a treaty signed between the United Kingdom of Great Britain and Ireland, Kingdom of France, Spain and Kingdom of Portugal on 22 April 1834, by which the four States undertook to expel from Portugal the Portuguese Infante Miguel and the Spanish Infante Carlos. With the beginning of the Carlist War, additional articles were signed in August of that year, by which the rest of the signatory parties undertook to help the legitimist government in Spain. The treaty was understood by the Austrian Empire, Russian Empire and Kingdom of Prussia, absolutist powers, as a joint diplomatic action in international politics to defend the liberal models represented by the governments of the Quadruple Alliance.
In the practical field, given that France and the United Kingdom were the first European powers, it was a question of both securing partial control of Spain and Portugal as medium-sized countries with unstable policies, such as the government of Isabella II in Spain, in a manner closer to a protectorate. This put an end to Spain's membership of the Holy Alliance, already quite denaturalized, and it was a significant milestone that two countries traditionally at loggerheads, such as France and the United Kingdom, reached a mutual understanding.
The Quadruple Alliance guaranteed the support of France and the United Kingdom for the dynastic pretensions of the daughter of Ferdinand VII of Spain, Isabella II, against the pretender to the Crown, Carlos María Isidro de Borbón, a fact that was significant for the defeat of the latter's supporters in the First Carlist War and for the consolidation of the regime.