August 31, 1839, marks the symbolic date–the Convention of Bergara–by which the First Carlist War came to an end. A peace agreement had actually already been signed between the opposing sides, the Liberals and the Carlists, on August 29; but two days later a public event was held in Bergara, Gipuzkoa, “the Bergara embrace” between Baldomero Espartero and Rafael Maroto, to proclaim peace. This remains a key date in Basque history because from this moment on the distinct administrative rights and liberties (the foruak or fueros) of the different Basque provinces would be called into question in increasingly centralizing efforts to make these provinces conform to a new state framework unfolding in Spain throughout the nineteenth century.
For Joseba Agirreazkuenaga, in The Making of the Basque Question (p. 174):
Article 1 of the convention made reference to the foral question—the political issue of inserting the Basque Provinces into the liberal constitutional framework established in the Spanish state between 1833 and 1837. The remaining points dealt with questions concerning the combatants, particularly those of the Carlist side. Not all of the Carlists accepted the terms of the convention but it was enough that it should receive the support of some battalions for the military fronts to collapse, forcing the remainder to flee into exile. The war did not, however, end automatically, but without the support of the Basques the Carlist dynastic option no longer appeared to have any possibility of success.
If you’d like to learn more about the First Carlist War, check out The Most Striking Events of a Twelvemonth’s Campaign with Zumalacarregui in Navarre and the Basque Provinces, by C.F. Henningsen, the recollections of an English adventurer who fought in the Carlist ranks. Besides being a robust and lively account of the course of the war itself, with no attention to detail spared, what makes the book equally interesting is Henningsen’s thoughts on the Basque Country (including detailed descriptions of a preindustrial Basque landscape) and on Basque culture in general.