The Battle of Trafalgar (21 October 1805) was a naval engagement between the British Royal Navy and the combined fleets of the French and Spanish Navies during the War of the Third Coalition (AugustDecember 1805) of the Napoleonic Wars (18031815).As part of Napoleon's plans to invade England, the French and Spanish fleets combined to take control of the English Channel and provide the Grande Arme safe passage. The allied fleet, under the command of the French admiral, Pierre-Charles Villeneuve, sailed from the port of Cdiz in the south of Spain on 18 October 1805. They encountered the British fleet under Lord Nelson, recently assembled to meet this threat, in the Atlantic Ocean along the southwest coast of Spain, off Cape Trafalgar.

Nelson was outnumbered, with 27 British ships of the line to 33 allied ships including the largest warship in either fleet, the Spanish Santsima Trinidad. To address this imbalance, Nelson sailed his fleet directly at the allied battle line's flank, hoping to break it into pieces. Villeneuve had worried that Nelson might attempt this tactic but, for various reasons, had made no plans in case this occurred. The plan worked almost perfectly; Nelson's columns split the Franco-Spanish fleet in three, isolating the rear half from Villeneuve's flag aboard Bucentaure. The allied vanguard sailed off while it attempted to turn around, giving the British temporary superiority over the remainder of their fleet. The ensuing fierce battle resulted in 22 allied ships being lost, while the British lost none.

The tactic exposed the leading ships in the British lines to intense fire from multiple ships as they approached the Franco-Spanish lines. Nelson's own HMS Victory led the front column and was almost knocked out of action. Nelson was shot by a French musketeer and died shortly before the battle ended. Villeneuve was captured along with his flagship Bucentaure. He attended Nelson's funeral while a captive on parole in Britain. The senior Spanish fleet officer, Admiral Federico Gravina, escaped with the remnant of the Franco-Spanish fleet (a third of what it had been in number of ships); he died of wounds sustained during the battle five months later.

The victory confirmed the naval supremacy Britain had established during the course of the eighteenth century, and was achieved in part through Nelson's departure from prevailing naval tactical orthodoxy.

The Napoleonic Wars (1803–1815) were a series of major global conflicts pitting the French Empire and its allies, led by Napoleon I, against a fluctuating array of European states formed into various coalitions. It produced a period of French domination over most of continental Europe. The wars stemmed from the unresolved disputes associated with the French Revolution and its resultant conflict. The wars are often categorised into five conflicts, each termed after the coalition that fought Napoleon: the Third Coalition (1805), the Fourth (1806–07), the Fifth (1809), the Sixth (1813–14), and the Seventh (1815).

Napoleon, upon ascending to First Consul of France in 1799, had inherited a republic in chaos; he subsequently created a state with stable finances, a strong bureaucracy, and a well-trained army. In December 1805 Napoleon achieved what is considered his greatest victory, defeating the allied Russo-Austrian army at Austerlitz. At sea, the British severely defeated the joint Franco-Spanish navy in the Battle of Trafalgar on 21 October 1805. This victory secured British control of the seas and prevented the invasion of Britain. Concerned about increasing French power, Prussia led the creation of the Fourth Coalition with Russia, Saxony, and Sweden, which resumed war in October 1806. Napoleon quickly defeated the Prussians at Jena and the Russians at Friedland, bringing an uneasy peace to the continent. The peace failed, though, as war broke out in 1809, with the badly prepared Fifth Coalition, led by Austria. At first, the Austrians won a stunning victory at Aspern-Essling, but were quickly defeated at Wagram, which was the bloodiest battle in history until the battle of Leipzig.

Hoping to isolate and weaken Britain economically through his Continental System, Napoleon launched an invasion of Portugal, the only remaining British ally in continental Europe. After occupying Lisbon in November 1807, and with the bulk of French troops present in Spain, Napoleon seized the opportunity to turn against his former ally, depose the reigning Spanish royal family and declare his brother King of Spain in 1808 as José I. The Spanish and Portuguese revolted with British support and expelled the French from Iberia in 1814 after six years of fighting.

Concurrently, Russia, unwilling to bear the economic consequences of reduced trade, routinely violated the Continental System, prompting Napoleon to launch a massive invasion of Russia in 1812. The resulting campaign ended in disaster for France and the near destruction of Napoleon's Grande Armée.

Encouraged by the defeat, Austria, Prussia, Sweden, and Russia formed the Sixth Coalition and began a new campaign against France, decisively defeating Napoleon at Leipzig in October 1813 after several inconclusive engagements. The Allies then invaded France from the east, while the Peninsular War spilled over into southwestern France. Coalition troops captured Paris at the end of March 1814 and forced Napoleon to abdicate in April. He was exiled to the island of Elba, and the Bourbons were restored to power. But Napoleon escaped in February 1815, and reassumed control of France for around one hundred days. After forming the Seventh Coalition, the allies defeated him at Waterloo in June 1815 and exiled him to the island of Saint Helena, where he died six years later.The Congress of Vienna redrew the borders of Europe and brought a period of relative peace. The wars had profound consequences on global history, including the spread of nationalism and liberalism, the rise of Britain as the world's foremost naval and economic power, the appearance of independence movements in Latin America and subsequent decline of the Spanish and Portuguese Empires, the fundamental reorganization of German and Italian territories into larger states, and the introduction of radically new methods of conducting warfare, as well as civil law. After the end of the Napoleonic Wars there was a period of relative peace in continental Europe, lasting until the Crimean War in 1853.