The reinstallation of liberal constitutionalism in Spain in 1820 was the first major challenge to the conservative political system of the Restoration, with far-reaching effects on the European and American continents. Spanish liberals endeavored to continue a political program based on the constitution of Cádiz (1812), although they encountered many obstacles in the form of inner discord and, especially, a local robust counterrevolutionary movement which enjoyed external support. The Trieno Constitucional was a moment of rapid and intense politicization, which surpassed the social elites to reach into a wider public that included popular sectors. A vigorous public opinion was developed and elections with mass participation were held at various administrative levels. The reverberations of the Spanish revolution were felt abroad. Naples-Sicily, Portugal and Piedmont-Sardinia soon adopted the Cádiz constitution. Moreover, during the Triennium the American possessions of the Spanish and Portuguese monarchies secured their independence, at a critical moment in which the Iberian metropoles were unable to delay the process anymore. Ultimately, the return of liberalism to Southern Europe was short-lived. The constitutional regimes fell after interventions accorded by the continental powers endorsed the local reactionary interests.