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The revolutions of 1848-9 failed because the foreign powers were too powerful and were easily able to overpower them to stop the revolutions.
In 1848, there was an uprising in Palermo, Sicily. This is because King Ferdinand refused to set up a provisional government. This spurred on uprisings in Naples by the secret societies, which forced King Ferdinand to make concessions. This lead to King Ferdinand introducing a parliament but its power was very limited.
In Lombardy, Austrian rulers refused to give in to and grant a reform. The Milanese stopped smoking in protest, this affected Austria as they collected the revenue from the cigarettes that were sold. This lead to demonstrations taking place in Venice demanding the release of Manin. Once Manin was released he declared a Venetian republic in Venice. As a result, Grand Duke Leopald granted a conservative constitution in Tuscany. Metternich resigned over the demonstrations for reform in Vienna, and in the same month, King Ferdinand was announced to be no longer then king after the Sicilian elections. After the battle in Milan, where Radetzky withdrew his troops, there was a political vacuum.
A provisional government was formed, and they asked for Charles Albert for protection. In the same month, Charles Albert introduced the Statuto in Piedmont, which lead to a constitutional monarchy. Charles Albert saw that Lombardy was available for annexation, so he declares war on Austria and invades Lombardy, where by troops from all over Italy join him. The Pope grants a limited constitution, which lets all Italian Catholics know that he is ok with them helping Charles Albert invade Lombardy.
However in April the Pope issues an Allocution condemning the war, as he saw Charles Albert as an aggressor that did he did not support a united Italy. This lead too many soldiers turning back from the war as they did not want to go against the Pope, as it would be going against their religious beliefs.
In May, Charles Albert took Peschiera, however a French army amassed on the border. Radetzky persuades Austria to fight for Lombardy. This lead to Charles Albert being dispelled from Lombardy. In November, a new PM, Rossi, was appointed in Austria, this puts a stop to any hopes of making any territorial claim to Piedmont. This lead to Austria taking back control of Lombardy. Also, November Rossi was murdered, which lead the Pope to flee the Papal States, which in turn leads to a revolutionary government, led by Galletti.
So in 1848, all Italian states apart from Lombardy were under a new government. This would not have happened if it wasn’t for Austria’s power and control over the states.
Similarly in 1849, Austria kept their power over the Italian states. In the Papal States, the parliament met for the first time. Mazzini was welcomed back to Rome, and told to choose 2 people to run Rome with him. Meanwhile Charles Albert was encouraged to try to overthrow Austrian power in Piedmont. However Radetzky crushed Piedmont at Novera. Charles Albert resigned and Emmanuel II was appointed king. The French sent an army to crush Roman Republic in the Papal States; though Garibaldi blocked the French troops for 2 months, at the same time Leopald returned to Tuscany after Charles Albert’s defeat. By June 1849, the French had 20,000 troops in Rome. This lead to Garibaldi Fleeing to San Marino, and Mazzini to London.
In August the Main and the Venice republic surrendered to Austrian control. A piece treaty was also signed by Piedmont, which left them embarrassed. This showed that Austria could not be defeated without foreign help.
In conclusion the failure of the revolutions was due to the interventions of foreign powers, because without Austria none of the Italian states would have been able to regain control of their governments and take back the power. However it was not due to the intervention of foreign powers, as it would have needed the help of foreign help to overthrow Austria to gain absolute control over Italy.