structural:expansion, information:highlighting
Prime Minister of Piedmont–Sardinia

Cavour formed a coalition with Urbano Rattazzi known as the Connubio ("marriage"), uniting the moderate men of the Right and of the Left, and brought about the fall of the d'Azeglio cabinet in November 1852. The King reluctantly accepted Cavour as prime minister, the most conservative possible choice, but their relationship was never an easy one.

Cavour was generally liberal and believed in free trade, freedom of opinion, and secular rule, but he was an enemy of republicans and revolutionaries, whom he feared as disorganized radicals who would upset the social order. Cavour dominated debate in Parliament but is criticized for the controversial methods he used while Prime Minister, including excessive use of emergency powers, employing friends, bribing some newspapers while suppressing others, and rigging elections, though these were fairly common practices for the time. The national debt soared by a factor of six because of his heavy spending on modernizing projects, especially railways, and building up the army and the Royal Sardinian Navy. When he became Prime Minister Piedmont had just suffered a major defeat by Austria, but when he died, Victor Emmanuel II ruled a state five times as large, which dominated Italy and ranked among Europe's great powers.

The allied powers of Britain and France asked Piedmont to enter the Crimean War, partially to encourage Austria to enter, which it would not do unless it was certain that Piedmontese troops were not available to attack Austrian positions in Italy. Cavour, who hoped that the allies would support Piedmont's expansion in Italy, agreed as soon as his colleagues' support would allow and entered the war on 10 January 1855. This was too late to truly distinguish themselves militarily, but the 18,000-man contingent earned Piedmont a position at the Congress of Paris that ended the war.

In January 1858, the Italian Felice Orsini 's attempted assassination of Napoleon III paradoxically opened an avenue of diplomacy between France and Piedmont. While in jail awaiting trial, Orsini wrote a public letter to the Emperor of the French, ending with, "Remember that, so long as Italy is not independent, the peace of Europe and Your Majesty is but an empty dream... Set my country free, and the blessings of twenty-five million people will follow you everywhere and forever." Orsini was still executed, but Napoleon III began to explore the possibility of a joint operation with Piedmont against Austria. Cavour and Napoleon met in July 1858 at Plombières-les-Bains, and the two agreed that Piedmont would attempt to provoke war with the Duchy of Modena, obliging Austria to enter, and France would then aid Piedmont. In return, Cavour reluctantly agreed to cede Savoy (the seat of the Piedmontese royal family) and the County of Nice to France, and also arranged a royal marriage between Princess Maria Clotilde of Savoy and Prince Napoléon-Jérôme Bonaparte, surprisingly without Victor Emmanuel's consent. In the same year, Cavour sent his cousin, the famous beauty, photographic artist, and secret agent Virginia Oldoïni, to further the interests of Italian unification with the emperor by whatever means possible, and by all accounts she succeeded, famously becoming the mistress of Napoleon.

Both France and Piedmont began to prepare for war, but diplomatic support diminished rapidly. Napoleon III quickly soured on the plot, and Britain, Prussia, and Russia proposed an international congress, with one likely goal the disarmament of Piedmont. Piedmont was saved by Austria's sending an ultimatum on 23 April, demanding that Piedmont disarm itself, thus casting Austria as an aggressor. France mobilised and slowly began to enter Italy, but Piedmont needed to defend itself for a short period. Fortunately, rainstorms and Austrian indecision under Ferenc Gyulay gave time for France to arrive in force.

The battles of Magenta and Solferino left Franco-Piedmontese forces in control of Lombardy, but the Austrians remained confident of defending their "fortress quadrilateral " area, with four fortresses in Verona, Legnago, Peschiera, and Mantua. These defences, the horrors of the Battle of Solferino, the possibility of Prussian entry into the war, and the potential for an over-strong Piedmontese state convinced Napoleon to sign a separate peace with Austria in the Armistice of Villafranca on 11 July 1859, ending the Second Italian War of Independence. Victor Emmanuel accepted the peace, but Cavour was so infuriated after reading the terms of the treaty that he tendered his resignation. He soon regained his optimism, however, as several of the terms, such as the restoration to power of the rulers of Tuscany and Modena, and the establishment of an Italian Confederation including Austria, were not actually carried out.

Alfonso Ferrero La Marmora succeeded to Cavour's post and insisted on following the treaty terms, even sending a letter to Tuscany asking that they restore their Grand Duke. Bettino Ricasoli, virtual dictator of Tuscany at the time, wrote about this appeal to his brother, saying: "Tell General La Marmora that I have torn his letter into a thousand pieces." France continued direct talks with Piedmont on the destiny of the central Italian states, all of whose provisional governments supported unification with Piedmont but were restrained by the treaty, which called for the restoration of their old rulers.

Cavour had retired to his estate at Leri, out of politics but concerned about the King's alliance with Garibaldi's revolutionaries and his desire to renew the war with Austria without allied support. When the weak La Marmora cabinet resigned, Victor Emmanuel was reluctant to have Cavour as premier again due both to their quarrel over the Armistice of Villafranca and Cavour's success in preventing the king from marrying his mistress after the queen's death. But Cavour was sent for on 20 January 1860.

Cavour agreed with Napoleon to cede Savoy and Nice to France, in order to annex Tuscany and Emilia to Piedmont. Plebiscites were arranged with huge majorities in all these provinces to approve the changes. Cavour managed to convince most that uniting Italy would make up for these territorial losses. With this, the first stage of unification was completed. It was now up to Garibaldi to overthrow the Bourbon Kingdom of the Two Sicilies and bring southern Italy into Piedmont's control.

Garibaldi was furious that his birthplace, Nice, had been ceded to France, and wished to recapture the city, but a popular insurrection in Palermo on 4 April 1860 diverted him southward. He requested a brigade of Piedmontese to take Sicily, but Cavour refused. So instead, Garibaldi raised a force of a thousand (I Mille) redshirt volunteers. They landed at Marsala in Sicily on 11 May and won the battles of Calatafimi and Milazzo, gaining control of Sicily. Cavour attempted to annex Sicily to Piedmont, but Garibaldi and his comrade Francesco Crispi would not allow it.

Cavour persuaded Victor Emmanuel to write a letter to Garibaldi, requesting that he not invade the mainland; the letter was indeed sent, but the King secretly wished Garibaldi to invade. He wrote another letter asking him to go ahead, but this was apparently never sent. Cavour meanwhile attempted to stir up a liberal revolution in Naples, but the populace was unreceptive. Garibaldi invaded, attempting to reach Naples quickly before Cavour found a way to stop him. On 7 September he entered Naples, at that time the largest city in Italy, and unilaterally declared Victor Emmanuel the King of Italy. Garibaldi was now military dictator of southern Italy and Sicily, and he imposed the Piedmontese constitution but publicly demanded that Cavour be removed, which alienated him slightly from Victor Emmanuel.

Garibaldi was unwilling to stop at this point, and planned an immediate invasion of the Papal States. Cavour feared France in that case would declare war to defend the Pope and successfully stopped Garibaldi from initiating his attack. Garibaldi had been weakened by the Battle of the Volturno, so Cavour quickly invaded the Papal regions of Umbria and Marche. This linked the territories conquered by Piedmont with those taken by Garibaldi. The King met with Garibaldi, who handed over control of southern Italy and Sicily, thus uniting Italy.

The relationship between Cavour and Garibaldi was always fractious: Cavour likened Garibaldi to "a savage" while Garibaldi memorably called Cavour "a low intriguer".