Napoleon III, known as "Louis Napoléon" prior to becoming Emperor, was the nephew of Napoleon I by his brother Louis Bonaparte, who married Hortense de Beauharnais, the daughter by the first marriage of Napoleon's wife Josephine de Beauharnais. TheEmpress Josephine proposed the marriage as a way to produce an heir for the Emperor, who agreed, as Josephine was by then infertile. Louis's paternity has been brought into question. Louis Bonaparte also harboured a lifelong suspicion about his legitimacy, although most historians have concluded that he was conceived by Louis Bonaparte and Hortense. During Napoleon I's reign, Louis-Napoléon's parents had been made king and queen of a French puppet state, the Kingdom of Holland. After Napoleon I's military defeats and deposition in 1815 and the restoration of the Bourbon monarchy in France, all members of the Bonaparte dynasty were forced into exile. The little Louis-Napoléon was brought up in Switzerland, living with his mother in Arenenberg Castle in the canton of Thurgau, and in Germany, receiving his education at the gymnasium school at Augsburg, Bavaria. As a young man he settled in Italy, where he and his elder brother Napoléon Louis espoused liberal politics and became involved with the Carbonari, an organization fighting Austria's domination of northern Italy. On March 17, 1831, while attempting to flee Italy due to a crackdown on revolutionary activity by Papal and Austrian troops, Louis-Napoléon's brother, suffering from measles, died in his arms. His experiences in Italy later had a profound effect on his foreign policy.

Meanwhile, France was under first the Bourbon and then the Orleanist monarchies. Under the latter emerged a Bonapartist movement that wanted to restore a Bonaparte to the throne. According to the law of succession Napoleon I had made when he was Emperor, the claim passed first to his son, the Duke of Reichstadt, known by Bonapartists as Napoleon II, or as "the King of Rome", the title his father had given him before the collapse of the Empire, a sickly youth living under virtual imprisonment at the court of Vienna. Next in line was Napoleon I's eldest brother Joseph Bonaparte, followed by Louis Bonaparte and his sons. Since Joseph had no male children, and because Louis-Napoléon's own elder brother had died in 1831, the death of the Duke of Reichstadt in 1832 made Louis-Napoléon the Bonaparte heir in the next generation. His uncle and father, relatively old men by then, left to him the active leadership of the Bonapartist cause. Thus he secretly returned to France in October 1836, for the first time since his childhood, to try to lead a Bonapartist coup at Strasbourg. Louis-Philippe had established the July Monarchy in 1830, and was confronted with opposition from the Legitimists, the Independents and the Bonapartists. The coup failed and Louis-Napoléon returned to Switzerland. When Louis-Philippe demanded his extradition, the Swiss refused to hand over a man who was a citizen and a member of their armed forces. In order to avoid a war, Louis-Napoléon left Switzerland of his own accord. Louis-Napoléon stayed at No. 6 Clarendon Square, Royal Leamington Spa, between 1838-1839. The building is now called Napoleon House and has a 'Blue plaque' put up by the local council.

He was quietly exiled to the United States of America, and spent four years in New York. He also sailed to Central America. He secretly returned to France and attempted yet another coup in August 1840, sailing with some hired soldiers into Boulogne. This time, he was caught and sentenced to life imprisonment, albeit in relative comfort, in the fortress of the town of Ham in the Department of the Somme. While in the Ham fortress, his eyesight reportedly became poor. During his years of imprisonment, he wrote essays and pamphlets that combined his claim to be emperor with progressive, mildly socialist economic proposals, which he came to define as Bonapartism. In 1844, his uncle Joseph died, making him the heir apparent to the Bonaparte claim. He finally escaped to Southport, England in May 1846 by changing clothes with a mason working at the fortress. His enemies would later derisively nickname him "Badinguet", the name of the mason whose identity he assumed. A month later, his father Louis died, making Louis-Napoléon the clear Bonapartist candidate to rule France. Louis-Napoléon lived within the borders of the United Kingdom until the revolution of February 1848 in France deposed Louis-Philippe and established a Republic. He was now free to return to France, which he immediately did. However, he found himself being asked to leave by the provisional government, which felt that he was an unnecessary distraction. Back in England, he volunteered to be a special constable in the event of Chartist rioting. In the same month, April, he ran for, and won, a seat in the assembly elected to draft a new constitution. He did not make a great contribution and, as a mediocre public orator, failed to impress his fellow members. Some even thought that, having lived outside of France almost all his life, he spoke French with a slight German accent. His temporary exile in 1848 proved to be a blessing in disguise for the December presidential election, as it meant he played no part in the June Days, and was able to enhance his image as "all things to all men" against his main opponent, Louis-Eugène Cavaignac, who had led the repression against the working-class of Paris.

When the constitution of the Second Republic was finally promulgated and direct elections for the presidency were held on 10 December 1848, Louis-Napoléon won a surprising landslide victory, with 5,587,759 votes (around 75% of the total); his closest rival, Cavaignac, received only 1,474,687 votes. The Monarchist right and much of the upper class supported him as the "least bad" candidate, as a man who would restore order, end the instability in France which had continued since the overthrow of the monarchy in February, and prevent a proto-communist revolution (in the vein of Friedrich Engels). A good proportion of the industrial working class, on the other hand, were won over by Louis-Napoléon's vague indications of progressive economic views. His overwhelming victory was above all due to the support of the non-politicized rural masses, to whom the name of Bonaparte meant something, as opposed to the other, little-known contenders. Louis-Napoléon's platform was the restoration of order after months of political turmoil, strong government, social consolidation, and national greatness, to which he appealed with all the credit of his name, that of France's national hero, Napoleon I, who in popular memory was credited with raising the nation to its pinnacle of military greatness and establishing social stability after the turmoil of the French Revolution. During his term as President, Louis-Napoléon Bonaparte styled himself the Prince-President (Le Prince-Président).

Despite his landslide victory, Louis-Napoléon was faced with a Parliament dominated by monarchists, who saw his government only as a temporary bridge to a restoration of either the House of Bourbon or of Orléans. Louis-Napoléon governed cautiously during his first years in office, choosing his ministers from among the more "centre-right" Orleanist Parti de l'Ordre monarchists, and generally avoiding conflict with the conservative assembly. He courted Catholic support by assisting in the restoration of the Pope's temporal rule in Rome, although he tried to please secularist conservative opinion at the same time by combining this with peremptory demands that the Pope introduce liberal changes to the government of the Papal States, including appointing a liberal government and establishing the Code Napoleon there, which angered the Catholic majority in the assembly. He soon made another attempt to gain Catholic support, however, by approving the Loi Falloux in 1851, which restored a greater role for the Church in the French educational system.

In the third year of his four-year mandate, President Louis-Napoléon Bonaparte asked the National Assembly for a revision of the constitution to enable the president to run for re-election, arguing that four years were not enough to implement his political and economic program fully. The Constitution of the Second Republic stated that the Presidency of the Republic was to be held for a single term of four years, with no possibility of re-election, a restriction written in the Constitution for fear that a President would abuse his power to transform the Republic into a dictatorship with a president for life. The National Assembly, dominated by monarchists who wished to restore the Bourbon dynasty, refused to amend the Constitution. The National Assembly law placed restrictions on universal male suffrage, imposing a three-year residency requirement. It prevented a large proportion of the lower class, which was itinerant, from voting. In spite of his limited powers forcing him to acquiesce to this law, Louis-Napoléon was able to seize the opportunity and break with the Assembly and the conservative ministers opposing his projects in favour of the dispossessed. He surrounded himself with lieutenants completely loyal to him, such as Morny and Persigny, secured the support of the army, and toured the country making populist speeches condemning the assembly and presenting himself as the protector of universal male suffrage.

After months of stalemate, and using the money of his mistress, Harriet Howard, he staged a coup d'état and seized dictatorial powers on 2 December 1851, the 47^th anniversary of Napoleon I's crowning as Emperor, and also the 46^th anniversary of the famous Battle of Austerlitz (hence another of Louis-Napoléon's nicknames: "The Man of December", "l'homme de décembre"). The coup was later declared to have been approved by the French people in a national referendum, the fairness and legality of which has been questioned by Napoleon III's detractors ever since. The coup of 1851 definitely alienated the reactionary and careerist elements in the Assembly. Victor Hugo, who had hitherto shown support for Louis-Napoléon Bonaparte, decided to go into exile after the coup, and became one of the harshest critics of Napoleon III, despite the amnesty of political opponents in 1859. New constitutional statutes were passed which officially maintained an elected Parliament and re-established universal male suffrage. However, the Parliament now became irrelevant as real power was completely concentrated in the hands of Louis-Napoléon and his bureaucracy. Exactly one year after the coup, on 2 December 1852, after approval by another referendum, the Second Republic was officially ended and the Empire restored, ushering in the Second French Empire. President Louis-Napoléon Bonaparte became Emperor Napoleon III. The numbering of Napoleon's reign treats Napoleon II, who never actually ruled, as a true Emperor (he had been briefly recognized as emperor from 22 June to 7 July 1814). That same year, Napoleon III began shipping political prisoners and criminals to penal colonies such as Devil's Island or (in milder cases) New Caledonia.

The emperor, hitherto a bachelor, began quickly to look for a wife to produce a legitimate heir. Most of the royal families of Europe were unwilling to marry into the parvenu Bonaparte family, and after rebuffs from Princess Carola of Sweden and from Queen Victoria's German niece Princess Adelheid of Hohenlohe-Langenburg, Napoleon decided to lower his sights somewhat and "marry for love", choosing the Countess of Teba, Eugénie de Montijo, a Spanish noblewoman of partial Scottish ancestry who had been brought up in Paris. In 1856, Eugenie gave birth to a legitimate son and heir, Louis Napoléon, the Prince Impérial. On 28 April 1855 Napoleon survived an attempted assassination. On 14 January 1858 Napoleon and his wife escaped another assassination attempt, plotted by Felice Orsini.

Until about 1861, Napoleon's regime exhibited decidedly authoritarian characteristics, using press censorship to prevent the spread of opposition, manipulating elections, and depriving the Parliament of the right to free debate or any real power. In the decade of the 1860s, Napoleon III made more concessions to placate his liberal opponents. This change began by allowing free debates in Parliament and public reports of parliamentary debates, continued with the relaxation of press censorship, and culminated in the appointment of the Liberal Émile Ollivier, previously a leader of the opposition to Napoleon's regime, as (effectively) Prime Minister in 1869. This later period is described by historians as the Liberal Empire. The French economy was rapidly modernized under Napoleon III, who desired a legacy as a reform-minded social engineer. The industrialization of France during this period, in general, appealed to members of both the business interests and the working classes. Downtown Paris was renovated with the clearing of slums, the widening of streets, and the construction of parks according to Baron Haussmann's plan. Working class neighbourhoods were moved to the outskirts of Paris, where factories utilized their labour. Some of his main backers were Saint-Simonians, and these supporters described Napoleon III as the "socialist emperor." Saint-Simonians at this time founded a new type of banking institution, the Crédit Mobilier, which sold stock to the public and then used the money raised to invest in industrial enterprises in France. This sparked a period of rapid economic development. Napoleon's Empire has been said to be the first regime in France to give "distinct priority to economic objectives". Napoleon sought to advance his belief in free trade, cheap credit, and the need to develop infrastructure as ways of ensuring progress and prosperity through government policy. Napoleon, like Haussmann and Pesigny, believed that the budget deficits that the state incurred due to its high contributions would be offset by subsequent high profits. His regime has also been cited as one of the few in French history to make a concerted effort towards breaking down trade barriers. As it turned out, this time period was favourable for industrial expansion. The gold rush in California, and later Australia, increased the European money supply. In the early years of the Empire, the economy also benefited from the coming of age of those born during the baby boom of the Restoration period. The steady rise of prices caused by the increase of the money supply encouraged company promotion and investment of capital. The mileage of railways in France increased from 3,000 to 16,000 kilometres during the 1850s, and this growth of railways allowed mines and factories to operate at higher rates of productivity. The 55 smaller rail lines of France were merged into 6 major lines, while new iron steamships replaced wooden ships. Between 1859 and 1869, a French company built the Suez Canal, opening a new chapter in global transportation and trade.

Algeria had been under French rule since 1830. Compared to previous administrations, Napoleon was far more sympathetic to the native Algerians, who appealed to his romantic sentiments. Because of this he halted European migration inland, restricting them to the coastal zone. Moreover, he freed the Algerian rebel leader Abd al Qadir (who had been promised freedom on surrender but was imprisoned by the previous administration) and gave him a stipend of 150,000 francs. He also allowed Muslims to serve in the military and civil service on theoretically equal terms and allowed them to migrate to France. In addition, he gave the option of citizenship; however, for Muslims to take this option they had to accept all of the French civil code, including parts governing inheritance and marriage which might conflict with Muslim tradition, and they had to reject the competence of religious courts. This was interpreted by some Muslims as requiring them to give up parts of their religion to obtain citizenship and was resented. One of the most influential decisions Louis Napoleon made in Algeria was to change its system of land tenure. While ostensibly well-intentioned, in effect this move destroyed the traditional system of land management and deprived many Algerians of land. While Napoleon did renounce state claims to tribal lands, he also began a process of dismantling tribal land ownership in favour of individual land ownership over the course of three generations, though this process was accelerated by later administrations. This process was corrupted by French officials sympathetic to French in Algeria who took much of the land they surveyed into public domain; in addition many tribal leaders, chosen for loyalty to the French rather than influence in their tribe, immediately sold communal land for cash.

In his speech at Bordeaux in 1852, Napoleon III famously proclaimed that "The Empire means peace" ("L'Empire, c'est la paix", literally 'The Empire, it is peace'), reassuring foreign governments that the new Emperor Napoleon would not attack other European powers in order to extend the French Empire. He was, however, thoroughly determined to follow a strong foreign policy to extend France's power and glory, and warned that he would not stand by and allow another European power to threaten its neighbour. He was also a partisan of a "policy of nationalities" (literally "politique de nationalités") re-casting the map of Europe, sweeping away small principalities to create unified nation-states, even when this seemed to have little relevance to France's material interests. In this he remained influenced by the themes of his uncle's policy, as related in the Mémorial de Sainte Hélène, such as Italian unification and a united Europe. These two factors led Napoleon to a certain adventurism in foreign policy, in the opinion of some contemporaries, although this was tempered by pragmatism.
Napoleon's challenge to Russia's claims to influence in the Ottoman Empire led to France's successful participation in the Crimean War (March 1854–March 1856). During this war Napoleon established a French alliance with Britain, which continued after the war's close. The defeat of Russia and the alliance with Britain gave France increased authority in Europe. This was the first war between European powers since the close of the Napoleonic Wars and the Congress of Vienna, marking a breakdown of the alliance system that had maintained peace for nearly half a century. The war also effectively ended the Concert of Europe and the Quadruple alliance, or "Waterloo Coalition" that the other four powers had established. The Paris Peace Conference of 1856 represented a high-water mark for the regime in foreign affairs, when Napoleon had followed through with his ideas set out in Les Idées Napoleoniennes.

In 1857, Napoleon III provided his assistance in negotiations to end the Anglo-Persian War, leading to the March 1857 Treaty of Paris. In East Asia, Napoleon took the first steps to establishing a French colonial influence in Indochina. He approved the launching of a naval expedition under Charles Rigault de Genouilly in 1858 to punish the Vietnamese for their mistreatment of French Catholic missionaries and force the court to accept a French presence in the country. An important factor in his decision was the belief that France risked becoming a second-rate power by not expanding its influence in East Asia. Also, the idea that France had a civilizing mission was spreading. This eventually led to a full-out invasion in 1861. By 1862 the war was over and Vietnam conceded three provinces in the south, called by the French Cochin-China, opened three ports to French trade, allowed free passage of French warships to Cambodia (which led to a French protectorate over Cambodia in 1863), allowed freedom of action for French missionaries and gave France a large indemnity for the cost of the war. France did not intervene, however, in the Christian-supported Vietnamese rebellion in Bac Bo, despite the urging of missionaries, or in the subsequent slaughter of thousands of Christians after the rebellion.

In China, France took part in the Second Opium War along with the United Kingdom, and in 1860 French troops entered Beijing. China was forced to concede more trading rights, allow freedom of navigation of the Yangtze, give full civil rights and freedom of religion to Christians, and give France and Britain a huge indemnity. This combined with the intervention in Vietnam set the stage for further French influence in China leading up to a sphere of influence over parts of southern China.

In 1866, French naval troops attacked Korea in response to the execution of French missionaries there. Though the campaign against Korea was primarily the work of the ranking French diplomat in China and not formally authorized by the French government, its failure nevertheless resulted in the decline of French influence in the region. In 1867, a military mission to Japan played a key role in modernizing the troops of the Shogun Tokugawa Yoshinobu, and even participated on his side against Imperial troops during the Boshin war.

As President of the Republic, Louis-Napoléon sent French troops to help restore Pope Pius IX as ruler of the Papal States in 1849 after his rule had been overthrown by the revolutionaries led by Giuseppe Mazzini and Giuseppe Garibaldi who had proclaimed the Roman Republic (although as a Carbonaro he had been involved in plotting a similar revolt in the Papal States during his youth in Italy). This won him support in France from Catholics (although many remained supporters of the Bourbon monarchy at heart). Yet at the same time he had sent an emissary to negotiate with the revolutionary Italian nationalist Mazzini. Napoleon remained attached to the ideal of Italian nationalism which he had embraced in his youth, and wished particularly to end Austrian rule in Lombardy and Venice. As Emperor, Napoleon dreamed of doing this, and thus satisfying his own inclinations and winning over liberal and left-wing opinion in France (which was passionately in favour of Italian unification) while at the same time supporting the Pope in Rome and thus maintaining conservative and Catholic support in France. These contradictory desires were evident in his policy in Italy. In April-July 1859 Napoleon made a secret deal at Plombières-les-Bains with Cavour, Prime Minister of Piedmont, for France to assist in expelling Austria from the Italian peninsula and bringing about a united Italy, or at least a united northern Italy, in exchange for Piedmont ceding to France Savoy and the Nice region (which was destined to become the so-called French Riviera). He went to war with Austria in 1859 and won victories at Magenta and Solferino, which resulted in the ceding of Lombardy to Piedmont by Austria (and in return received Savoy and Nice from Piedmont as promised in 1860). After this had been done, however, Napoleon decided to end French involvement in the war. This early withdrawal, however, failed to prevent central Italy, including most of the Papal states, being incorporated into the new Italian state. This led Catholics in France to turn against Napoleon. Napoleon tried to redress the damage by maintaining French troops in the city of Rome itself, which prevented the Italian government seizing it from the Pope, a policy which Napoleon's devoutly Catholic wife Eugenie fervently supported. However, Napoleon on the whole failed to win back Catholic support at home (and made moves to appeal instead to the anti-Catholic left in his domestic policy in the 1860s, most notably by appointing the anti-clerical Victor Duruy Minister for Education, who further secularised the schooling system). Nonetheless, French troops remained in Rome to protect the Pope until the outbreak of the Franco-Prussian War in 1870.

Napoleon III envisioned a "Grand Scheme for the Americas," which would consist of three general points. The first involved recognition of the Confederate States of America and a military alliance with them. The second involved reintroducing monarchical rule to Latin America, in the form of Maximillian I in Mexico, and increasing French trade throughout Latin America. The third and final point involved control over Mexico with the creation of a large buffer state from the Rio Grande to the Californian Baja.

Another example of Napoleon's adventurism in foreign policy was the French intervention in Mexico (January 1862–March 1867). Napoleon, using as a pretext the Mexican Republic's refusal to pay its foreign debts, planned to establish a French sphere of influence in North America by creating a French-backed monarchy in Mexico, a project that was supported by Mexican conservatives who resented the Mexican Republic's laicism. The United States was unable to prevent this contravention of the Monroe Doctrine because of the American Civil War; Napoleon hoped that the Confederates would be victorious in that conflict, believing they would accept the new regime in Mexico. But his imperial dreams would not be so easy to achieve. In Mexico the French army suffered its first military defeat in 50 years, on the Fifth of May, 1862 in Puebla when the Mexican army under the leadership of General Ignacio Zaragoza defeated a much better-equipped French army. The defeat not only surprised the world, but served to revitalize the national spirit of Mexicans, helping to sustain a guerrilla warfare that lasted 5 years. In the end, it remained the Second Mexican Empire. With the support of Mexican conservatives and French troops, in 1863 Napoleon installed Maximilian I of Mexico, a Habsburg prince, as emperor. Ruling President Benito Juarez and his Republican forces retreated to the countryside and fought against the French troops and the Mexican monarchists. The combined Mexican monarchist and French forces won victories up until 1865, but then the tide began to turn against them, in part because the American Civil War had ended. The U.S. government was now able to give practical support to the Republicans, supplying them with arms and establishing a naval blockade to prevent French reinforcements arriving from Europe. Due to continued losses inflicted by the Mexican guerrillas loyal to the Republic and the threat of an American military intervention, Napoleon withdrew French troops from Mexico in 1866, which left Maximilian and the Mexican monarchists doomed to defeat in 1867. Despite Napoleon's pleas that he abdicate and leave Mexico, Maximilian refused to abandon the Mexican conservatives who had supported him, and remained alongside them until the bitter end, when he was captured by the Republicans and then shot on 19 June 1867. 

France saw its dominance on the continent of Europe eroded by Prussia's crushing victory over Austria in the Austro-Prussian War in June–August 1866. Due in part to his Carbonaro past, Napoleon was unable to ally himself with Austria, despite the obvious threat that a victorious Prussia would pose to France. Napoleon felt secure in the presumption that the war with Austria would be drawn out, or would result in Austrian victory, when he agreed not to intervene in 1864. Yet, having decided not to prevent the Prussian rise to power by allying against her, Napoleon also failed to take the opportunity to demand Prussian consent to French territorial expansion in return for France's neutrality. Napoleon only requested that Prussia agree to French annexation of Belgium and Luxembourg after Prussia had already defeated Austria, by which time France's neutrality was no longer needed by Prussia. This extraordinary foreign policy failure saw France gain nothing while allowing Prussia's strength to increase greatly. In part the reason for the Emperor's blunder must be laid on his deteriorating health during this period—he had begun to suffer from a bladder stone that caused him great pain, even preventing him from riding a horse. Napoleon's later attempt in 1867 to re-balance the scales by purchasing Luxembourg from its ruler, William III of the Netherlands, was thwarted by a Prussian threat of war. The Luxembourg Crisis ended with France renouncing any claim to Luxembourg in the Treaty of London (1867). Napoleon III paid the price for his failure to help defend Austria from Prussia in 1870 when, goaded by the diplomacy of the Prussian Prime Minister (and chancellor of the North German Confederation, and soon of the new German Empire) Otto von Bismarck, he began the Franco-Prussian War. This war proved disastrous for France, and was instrumental in giving birth to the German Empire, which would take France's place as the major land power in continental Western Europe until the end of World War I. In battle against Prussia in July 1870 the Emperor was captured at the Battle of Sedan (2 September) and was deposed by the forces of the Third Republic in Paris two days later.

Napoleon spent the last few years of his life in exile in England, with Eugenie and their only son. The family lived at Camden Place Chislehurst, (then in Kent), where he died on 9 January 1873. He was haunted to the end by bitter regrets and by painful memories of the battle at which he lost everything; Napoleon's last words, addressed to Dr. Henri Conneau standing by his deathbed, reportedly were, "Were you at Sedan?" ("Etiez-vous à Sedan?")

Napoleon was originally buried at St. Mary's, the Catholic Church in Chislehurst. However, after his son died in 1879 fighting in the British Army against the Zulus in South Africa, the bereaved Eugenie decided to build a monastery. The building would house monks driven out of France by the anti-clerical laws of the Third Republic, and would provide a suitable resting place for her husband and son. Thus, in 1888, the body of Napoleon III and that of his son were moved to the Imperial Crypt at St Michael's Abbey, Farnborough, Hampshire, England. Eugenie, who died many years later, in 1920, now rests there with them. It was reported in 2007 that the French Government was seeking the return of his remains to be buried in France, but that this is opposed by the monks of the abbey.