Napoleon III
Early years

On the evening of 14 January 1858, Napoleon and the Empress escaped an assassination attempt unharmed. A group of conspirators threw three bombs at the imperial carriage as it made its way to the opera. Eight members of the escort and bystanders were killed and over one hundred people injured. The culprits were quickly arrested. The leader was an Italian nationalist, Felice Orsini, who was aided by a French surgeon Simon François Bernard. They believed that if Napoleon III were killed, a republican revolt would immediately follow in France and the new republican government would help all Italian states win independence from Austria and achieve national unification. Bernard was in London at the time. Since he was a political exile, the Government of the United Kingdom refused to extradite him, but Orsini was tried, convicted and executed on 13 March 1858. The bombing focused the attention of France and particularly of Napoleon III, on the issue of Italian nationalism.

Part of Italy, particularly the Kingdom of Sardinia, was independent, but central Italy was still ruled by the Pope (in this era, Pope Pius IX), while Lombardy, Venice and much of the north was ruled by Austria. Other states were de jure independent (notably the Duchy of Parma and the Grand Duchy of Tuscany) but de facto fully under Austrian influence. Napoleon III had fought with the Italian patriots against the Austrians when he was young and his sympathy was with them, but the Empress, most of his government and the Catholic Church in France supported the Pope and the existing governments. The British Government was also hostile to the idea of promoting nationalism in Italy. Despite the opposition within his government and in his own palace, Napoleon III did all that he could to support the cause of Piedmont-Sardinia. The King of Piedmont-Sardinia, Victor Emmanuel II, was invited to Paris in November 1855 and given the same royal treatment as Queen Victoria.

Count Cavour, the Prime Minister of Piedmont-Sardinia, came to Paris with the King and employed an unusual emissary in his efforts to win the support of Napoleon III: his young cousin, Virginia Oldoini, Countess of Castiglione (1837–1899). As Cavour had hoped, she caught the Emperor's eye and became his mistress. Between 1855 and 1857, she used the opportunity to pass messages and to plead the Italian cause.

In July 1858, Napoleon arranged a secret visit by Count Cavour. In the Plombières Agreement they agreed to join forces and drive the Austrians from Italy. In exchange, Napoleon III asked for Savoy (the ancestral land of the King of Piedmont-Sardinia) and the then bilingual County of Nice, which had been taken from France after Napoleon's fall in 1815 and returned to Piedmont-Sardinia. Cavour protested that Nice was Italian, but Napoleon responded that "these are secondary questions. There will be time later to discuss them."

Assured of the support of Napoleon III, Count Cavour began to prepare the Royal Sardinian Army for war against Austria. Napoleon III looked for diplomatic support. He approached Lord Derby (the Prime Minister of the United Kingdom) and his government; Britain was against the war, but agreed to remain neutral. Still facing strong opposition within his own government, Napoleon III offered to negotiate a diplomatic solution with the twenty-eight-year-old Emperor Franz Joseph I of Austria in the spring of 1858. The Austrians demanded the disarmament of Piedmont-Sardinia first and sent thirty thousand soldiers to reinforce their garrisons in Italy. Napoleon III responded on 26 January 1859 by signing a treaty of alliance with Piedmont-Sardinia. Napoleon promised to send two hundred thousand soldiers to help one hundred thousand soldiers from Piedmont-Sardinia to force the Austrians out of Northern Italy; in return, France would receive the County of Nice and Savoy provided that their populations would agree in a referendum.

It was the Emperor Franz Joseph, growing impatient, who finally unleashed the war. On 23 April 1859, he sent an ultimatum to the government of Piedmont-Sardinia demanding that they stop their military preparations and disband their army. On 26 April, Count Cavour rejected the demands, and on 27 April, the Austrian army invaded Piedmont.
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