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The Revolutions of 1820 were a revolutionary wave in Europe. It included revolutions in Russia (Decembrist revolt), Spain, Portugal and Italy for constitutional monarchies, and for independence from Ottoman rule in Greece. Unlike the revolutionary wave in the 1830s, these tended to take place in the peripheries of Europe.
The 1820 revolution began in Sicily and in Naples, against King Ferdinand I of the Two Sicilies, who was forced to make concessions and promise a constitutional monarchy. This success inspired Carbonari in the north of Italy to revolt too. In 1821, the Kingdom of Sardinia (Piedmont) obtained a constitutional monarchy as a result of Carbonari actions, as well as other reforms of liberalism.
The Holy Alliance would not tolerate this state of affairs and decided in October 1820 to intervene. In February, 1821, it sent an army to crush the revolution in Naples. The King of Sardinia also called for Austrian intervention. Faced with an enemy overwhelmingly superior in number, the Carbonari revolts collapsed and their leaders fled into exile.
Colonel Rafael del Riego led a large part of the Spanish army in a mutiny, demanding that the liberal constitution of 1812 be restored. King Ferdinand VII agreed, but secretly asked for aid from the Congress system which, in the Congress of Verona of 1822, agreed to have France send 100,000 troops, which promptly defeated Riego's forces and reinstalled an absolute monarchy. This period was known as Trienio Liberal 
Beginning with a military insurrection in the city of Porto, in northern Portugal, in 1820 that quickly and peacefully spread to the rest of the country, the Revolution resulted in the return in 1821 of the Portuguese Court to Portugal from Brazil, where it had fled during the Peninsular War, and initiated a constitutional period in which the 1822 Constitution was ratified and implemented.
One should bear in mind that Brazil's and Portugal's histories were intertwined at that time.
According to Kenneth Maxwell, "the important point about Brazil is that it became economically and politically emancipated between 1808 and 1820 while acting as the centre of the Luso-Brazilian Empire", meaning Brazil's independence was proclaimed after the nation had had an "imperial-like" experience.
"This unusual circumstance explains why in 1820 it was Portugal that declared independence from Brazil, and only afterwards, that Brazil declared its independence from Portugal", as one may read in the Manifesto issued by the rebels in Oporto in 1820:
"[...] The idea of the status of a colony to which Portugal in effect is reduced afflicts deeply all those citizens who still conserve a sentiment of national dignity. Justice is administered from Brazil to the loyal people in Europe [...]"