Napoleon III (Charles Louis Napoléon Bonaparte; 20 April 1808 – 9 January 1873) was the first President of France (as Louis-Napoléon Bonaparte) from 1848 to 1852 and the Emperor of the French from 1852 to 1870. A nephew of Napoleon I, he was the last monarch to rule over France. Elected to the presidency of the Second Republic in 1848, he seized power by force in 1851, when he could not constitutionally be reelected; he later proclaimed himself Emperor of the French. He founded the Second Empire, reigning until the defeat of the French Army and his capture by Prussia and its allies at the Battle of Sedan in 1870. Napoleon III was a popular monarch, who oversaw the modernisation of the French economy and filled Paris with new boulevards and parks. He expanded the French overseas empire and made the French merchant navy the second largest in the world, engaged in the Second Italian War of Independence as well as the disastrous Franco-Prussian War, in which he commanded his soldiers during the fight and was captured.

Napoleon III commissioned a grand reconstruction of Paris carried out by the man he appointed as prefect of the Seine, Baron Georges-Eugène Haussmann. He expanded and consolidated the railway system throughout the nation and modernized the banking system. Napoleon III promoted the building of the Suez Canal and established modern agriculture, which ended famines in France and made the country an agricultural exporter. He negotiated the 1860 Cobden–Chevalier Free Trade Agreement with Britain and similar agreements with France's other European trading partners. Social reforms included giving French workers the right to strike, the right to organize, and the right to be admitted to a French University as a woman.

In foreign policy, Napoleon III aimed to reassert French influence in Europe and around the world. In Europe, he allied with Britain and defeated Russia in the Crimean War (1853–1856). His regime assisted Italian unification by defeating the Austrian Empire in the Franco-Austrian War and later annexed Savoy and Nice through the Treaty of Turin as its deferred reward. At the same time, his forces defended the Papal States against annexation by Italy. He was also favourable towards the 1859 union of the Danubian Principalities, which resulted in the establishment of the United Principalities of Moldavia and Wallachia. Napoleon III doubled the area of the French colonial empire with expansions in Asia, the Pacific and Africa. On the other hand, the intervention in Mexico, which aimed to create a Second Mexican Empire under French protection, ended in total failure. From 1866, Napoleon III had to face the mounting power of Prussia as its Chancellor Otto von Bismarck sought German unification under Prussian leadership. In July 1870, Napoleon III reluctantly declared war on Prussia after pressure by the public. The French Army was rapidly defeated as Napoleon III was captured at Sedan. He was swiftly dethroned and the Third Republic was proclaimed in Paris. He went into exile in England, where he died in 1873.