The Trienio Liberal (Spanish pronunciation: [ˈtɾjenjo liβeˈɾal], "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".