Background
Ever since the fall of the Roman Empire, Italy had been extensively divided from within by large nation states in the central and southern portion of the 'boot', and foreign-controlled city states in the north. At least a dozen individual governments ruled over the area of modern Italy. Only one nation had a native Italian dynasty, the house of Savoy; this was the Kingdom of Sardinia, or more commonly referred to as the nation of Piedmont. This internal strife and lack of a sense of Italian nationalism for many hundreds of years put a severe damper on the unification of the peninsula.
During the Napoleonic era, Italy enjoyed a brief sense of unity by being consolidated into two separate kingdoms and two other parts being directly controlled by France. The four separate parts, Piedmont, the Kingdom of Italy, the Kingdom of Naples, and the western shore from Florence in the north to Rome in the south. These kingdoms were under the rule of various "monarchs" imposed upon them by Napoleon. But after the Congress of Vienna, Italy was disorganized back into the way it was prior to Napoleon's invasion.
After Napoleon had thoroughly ravaged Europe and had finally been exiled the second time in 1815, nationalism became a wonderfully dangerous thing. Nationalism is pride in one's country, but in the 19th century, it also came to mean pride in one's ethnicity because of the great influence of Social Darwinism. Ethnicity became a rallying point for the middle class and peasantry of Europe for political and social gains. By 1848 the entirety of Europe suffered a few massive revolts in which monarchies were toppled left and right. In much of Europe the former heads of state were able to regain control of their countries but with severe restrictions.
In Italy, there was almost no respect for the existing governments by the populace and a great desire for a unified Italy grew. The movement was divided into two groups, one radical and one moderate. The moderate faction was lead (sorta) by Josef Mazzini, whose writings became the basis of the moral cause for unification. The radical faction was greatly divided, but the main figure head was Guiseppe Garibaldi. Both figures were very prominent with Italian secret societies like Young Italy.
The Nation of Piedmont and the brilliance of Cavour
After the confusion of 1848, the Kingdom of Sardinia, or Piedmont, as it is better known, arose as a constitutional monarchy with the one and only Italian monarch in Italy, King Victor Emmanuel of the House of Savoy. The king was just a figurehead however, for there was a great power behind the throne of the small nation. This power was Camillo di Cavour, and he easily rivals Bismark as the greatest politician of all time. Taking over as prime minister in 1852, Cavour quickly improved the nation, bringing railroads, better farming methods, and opened trade with other nations. Cavour practiced Bismark's idea of "politik," which was basically an "end justifies the means" way of thinking in the political arena. Only more harsh.
To achieve Italian unity, Cavour knew he would never be able to rely on a revolt from the people nor the secret societies that were very popular. Italian Unification would have to be achieved through fierce political combat to drive the Austrian Empire out, and then brutal war on the smaller powers of the Italian peninsula. To gain a foothold in the grand political arena of Europe, Cavour sent troops to Russia to participate in the Crimean War in 1854. His hope by doing this was to present the Italian Question to the higher powers of Europe in a hope that they will disagree and erupt in war over the issue. Cavour did just that.
At the time, Louis Napoleon Bonaparte was the ruler of France. Napoleon III was very supportive of Cavour's cause. Nappy 3 himself participated in underground organizations and even helped in a revolt in 1831 before he became the second French emperor. Nappy 3 was trying to spread French influence throughout Europe, so when Cavour tricked Austria to declare war on Piedmont, it was not long before the French came in to whoop on the Hapsburgs. It was a short war; two battles over two months. Both won by the Franco-Piedmontese forces. The war started in April, but by July, France was pressured into making peace with the Austrians because Prussian forces were gathering on the Rhine. The peace agreement was not made with Cavour's presence and as a result the peace treaty was not as much to his liking as it could have been. With France no longer in the war, Piedmont could not hope to win against the Austrians alone and Piedmont left the war with what was given to them in the Franco-Austrian agreement; the city state of Lombardy, but left Venetia in the hands of the Hapsburgs. Even worse, was the Franco-Austrian settlement constructed a sort of federal union, the members of which were all the secular leaders of Italy, and was presided over by the Pope.
In response to the Piedmontese war in April, and still raging after the peace settlements, Italian underground organizations staged revolts all over Italy. These revolts were successful in Tuscany, Modena, Parma, and Romagna; all of which were absorbed by Piedmont with treaties made by Cavour. The nation of Piedmont was now taking up half of the Northern portion of the boot, and about one third of the peninsula in all. To punish those that had revolted in Romagna, formerly a Papal State, the pope excommunicated all the organizers of the revolt, as well as the leaders of the enlarged Piedmont. Even though excommunication was a severe penalty in Catholic Italy, those involved remained true to their cause, and met for a parliamentry meeting. Despite the disapproval of the papacy, Britain and France both hailed the Piedmontese (ha. Like the papacy disapproving something would have stopped Britain from doing it... ).
There were now only four states in all of Italy, the nation of Piedmont, Venetia, the Papal States, and the Kingdom of the Two Sicilies. The kingdom of Two Sicilies was located in the southern half of Italy and guess where, Sicily. Two Sicilies was ruled by the Bourbons, but distant cousins of those of French fame. Just like those of French fame, however, these Bourbons were politically corrupt and faced a long history of political instability within the country.
In 1860 this problem was solved by Giuseppe Garibaldi. A prominent member of Italian secret societies, he organized 1,150 Red Shirts into what was called "Garibaldi's Thousand." He invaded Two Sicilies, and the government there collapsed around itself as thousands of citizens rallied behind Garibaldi.
Garibaldi then turned north to Rome, but was stopped by an army from Piedmont. Not because Piedmont wished to fight Garibaldi, but because if Garibaldi invaded Rome, then the French soldiers stationed there would defend it, and the trained and hardened French forces would probably easily defeat Garibaldi. Afterwards, the French would probably be forced by international pressure to place the Bourbons back on the throne of Two Sicilies. Cavour convinced Garibaldi that a constitutional monarchy was best, and Two Sicilies was annexed to Piedmont. Soon after, the Papal States excluding Rome were added to Piedmont by way of universal manhood vote. In 1861, the nation of Piedmont changed its name to the Kingdom of Italy. The only parts of Italy not part of the kingdom were Venetia and Rome. The former was annexed in 1866 for aide to the Prussians in a war against Austria. The latter was annexed when the French troops withdrew to help their homeland in the Franco-Prussian War in 1870.
Problems after Unification
In 1870 when Italian forces invaded Rome, the pope condemned the attack and those involved. He also put himself under a sort of house arrest and secluded himself within the Vatican. His successors also did this, up until 1929 when the Great Depression began. The pope was so anti-Italian unity that in order for an Italian Catholic to be a good Catholic, they would also have to be anti-unity. But in order to be a good patriot, the Catholic would have to be pro-unity and anti-pope.
Also, the parliament of Italy was at first horridly undemocratic, despite claiming to be democratic; only about 3% of Italy's population was allowed to vote. Because of this, some revolutionary agitation still existed, but not to the extent it had prior to 1860.
Sources:
Palmer, R.R. and Joel Colton. A History of the Modern World. 6th Edition. New York. 1950.