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Expedition against Rome

Garibaldi himself was intensely anti-Catholic and anti-papal. His efforts to overthrow the pope by military action mobilized anti-Catholic support. On 28 September 1862, 500 Irish labourers disrupted a pro-Garibaldi meeting in London by chanting "God and Rome", which triggered weeks of ethnic conflict between Protestants and Irish Catholics. Garibaldi's hostility to the pope's temporal domain was viewed with great distrust by Catholics around the world, and the French Emperor Napoleon III had guaranteed the independence of Rome from Italy by stationing a French garrison there. Victor Emmanuel was wary of the international repercussions of attacking Rome and the pope's seat there, and discouraged his subjects from participating in revolutionary ventures with such intentions. Nonetheless, Garibaldi believed he had the secret support of his government. Once he was excommunicated by the pope, he chose the Protestant pastor Alessandro Gavazzi as his army chaplain.

In June 1862, he sailed from Genoa to Palermo to gather volunteers for the impending campaign, under the slogan Roma o Morte ("Rome or Death"). An enthusiastic party quickly joined him, and he turned for Messina, hoping to cross to the mainland there. He arrived with a force of around two thousand, but the garrison proved loyal to the king's instructions and barred his passage. They turned south and set sail from Catania, where Garibaldi declared that he would enter Rome as a victor or perish beneath its walls. He landed at Melito di Porto Salvo on 14 August and marched at once into the Calabrian mountains.

Far from supporting this endeavour, the Italian government was quite disapproving. General Enrico Cialdini dispatched a division of the regular army, under Colonel Emilio Pallavicini, against the volunteer bands. On 28 August, the two forces met in the rugged Aspromonte. One of the regulars fired a chance shot, and several volleys followed, killing a few of the volunteers. The fighting ended quickly, as Garibaldi forbade his men to return fire on fellow subjects of the Kingdom of Italy. Many of the volunteers were taken prisoner, including Garibaldi, who had been wounded by a shot in the foot. The episode was the origin of a famous Italian nursery rhyme: Garibaldi fu ferito ("Garibaldi was wounded").

A government steamer took him to a prison at Varignano near La Spezia, where he was held in a sort of honourable imprisonment and underwent a tedious and painful operation to heal his wound. His venture had failed, but he was consoled by Europe's sympathy and continued interest. After he regained his health, the government released Garibaldi and let him return to Caprera.

En route to London in 1864 he stopped briefly in Malta, where many admirers visited him in his hotel. Protests by opponents of his anticlericalism were suppressed by the authorities. In London, his presence was received with enthusiasm by the population. He met the British Prime Minister Viscount Palmerston, as well as revolutionaries then living in exile in the city. At that time, his ambitious international project included the liberation of a range of occupied nations, such as Croatia, Greece, and Hungary. He also visited Bedford and was given a tour of the Britannia Iron Works, where he planted a tree (which was cut down in 1944 due to decay).