Trienio Liberal The Trienio Liberal ( , " Liberal Triennium " ) was a period of three years of liberal government in Spain . After the revolution of 1820 the movement spread quickly to the rest of Spain and the Spanish Constitution of 1812 was reinstated . The Triennium was a volatile period between liberals and conservatives in Spain , and constant political tensions between the two groups progressively weakened the government ' s authority . Finally in 1823 , with the approval of the crown heads of Europe , a French army invaded Spain and reinstated the King ' s absolute power . This invasion is known in France as the " Spanish Expedition " ( " expédition d ’ Espagne " ) , and in Spain as " The Hundred Thousand Sons of St . Louis " . The Revolution of Cabezas de San Juan . Spain ' s King Ferdinand VII provoked widespread unrest , particularly in the army , by refusing to accept the liberal Spanish Constitution of 1812 . The King sought to reclaim the Spanish colonies in the Americas that had recently revolted successfully , consequently depriving Spain of an important source of revenue . In January 1820 , soldiers assembled at Cadiz for an expedition to South America , angry over infrequent pay , bad food , and poor quarters , mutinied under the leadership of Colonel Rafael del Riego y Nuñez . Pledging fealty to the 1812 Constitution , they seized their commander . Subsequently the rebel forces moved to nearby San Fernando where they began preparations to march on the capital of Madrid . Liberal government . Despite the rebels ' relative weakness , Ferdinand accepted the constitution on March 9 , 1820 , granting power to liberal ministers and ushering in the so - called Liberal Triennium ( " el Trienio Liberal " ) , a period of popular rule . However , in this liberal atmosphere , political conspiracies of both right and left proliferated in Spain , as was the case across much of Europe . Liberal revolutionaries stormed the King ' s palace and seized Ferdinand VII who was a prisoner of the Cortes in all but name for the next three years . During this time Ferdinand retired to Aranjuez . The elections to the Cortes Generales in 1822 were won by Rafael del Riego . Ferdinand ' s supporters set themselves up at Urgell , took up arms and put in place an absolutist regency . Ferdinand ' s supporters , accompanied by the Royal Guard , staged an uprising in Madrid which was subdued by forces supporting the new government and its constitution . Despite the defeat of Ferdinand ' s supporters at Madrid , civil war erupted in the regions of Castile , Toledo , and Andalusia . French intervention and restoration of Absolutism . In 1822 , Ferdinand VII applied the terms of the Congress of Vienna , lobbied for the assistance of the other absolute monarchs of Europe , in the process joining the Holy Alliance formed by Russia , Prussia , Austria and France to restore absolutism . In France , the ultra - royalists pressured Louis XVIII to intervene . To temper their counter - revolutionary ardor , the Duc de Richelieu deployed troops along the Pyrenees Mountains along the France - Spain border , charging them with halting the spread of Spanish liberalism and the " yellow fever " from encroaching into France . In September 1822 this " " cordon sanitaire " " became an observation corps and then very quickly transformed itself into a military expedition . The Holy Alliance ( Russia , Austria and Prussia ) refused Ferdinand ' s request for help , but the Quintuple Alliance ( Britain , France , Russia , Prussia and Austria ) at the Congress of Verona in October 1822 gave France a mandate to intervene and restore the Spanish monarchy . On 22 January 1823 , a secret treaty was signed at the congress of Verona , allowing France to invade Spain to restore Ferdinand VII as an absolute monarch . With this agreement from the Holy Alliance , on 28 January 1823 Louis XVIII announced that " a hundred thousand Frenchmen are ready to march , invoking the name of Saint Louis , to safeguard the throne of Spain for a grandson of Henry IV of France " .