The War of the Third Coalition was a European conflict spanning the years 1805 to 1806. During the war, France and its client states under Napoleon I opposed an alliance, the Third Coalition, made up of the United Kingdom, the Holy Roman Empire, the Russian Empire, Naples, Sicily and Sweden. Prussia remained neutral during the war.

Britain had already been at war with France following the breakdown of the Peace of Amiens and remained the only country still at war with France after the Treaty of Pressburg. From 1803 to 1805, Britain stood under constant threat of a French invasion. The Royal Navy, however, secured mastery of the seas and decisively destroyed a Franco-Spanish fleet at the Battle of Trafalgar in October 1805.

The Third Coalition itself came to full fruition in 180405 as Napoleon's actions in Italy and Germany (notably the arrest and execution of the Duc d'Enghien) spurred Austria and Russia into joining Britain against France. The war would be determined on the continent, and the major land operations that sealed the swift French victory involved the Ulm Campaign, a large wheeling manoeuvre by the Grande Arme lasting from late August to mid-October 1805 that captured an entire Austrian army, and the decisive French victory over a combined Austro-Russian force under Tsar Alexander I at the Battle of Austerlitz in early December. Austerlitz effectively brought the Third Coalition to an end, although later there was a small side campaign against Naples, which also resulted in a decisive French victory at the Battle of Campo Tenese.

On 26 December 1805, Austria and France signed the Treaty of Pressburg, which took Austria out of both the war and the Coalition, while it reinforced the earlier treaties of Campo Formio and of Lunville between the two powers. The treaty confirmed the Austrian cession of lands in Italy and Bavaria to France and in Germany to Napoleon's German allies, imposed an indemnity of 40 million francs on the defeated Habsburgs, and allowed the defeated Russian troops free passage, with their arms and equipment, through hostile territories and back to their home soil. Victory at Austerlitz also prompted Napoleon to create the Confederation of the Rhine, a collection of German client states which pledged themselves to raise an army of 63,000 men. As a direct consequence of these events, the Holy Roman Empire ceased to exist when, in 1806, Francis II abdicated the Imperial throne, emerging as Francis I, Emperor of Austria. These achievements, however, did not establish a lasting peace on the continent. Austerlitz had driven neither Russia nor Britain, whose armies protected Sicily from a French invasion, to settle. Meanwhile, Prussian worries about the growing French influence in Central Europe sparked the War of the Fourth Coalition in 1806.

Napoleon Bonaparte (born Napoleone di Buonaparte; 15 August 1769 – 5 May 1821) was a French military and political leader who rose to prominence during the French Revolution and led several successful campaigns during the Revolutionary Wars. He was the de facto leader of the French Republic as First Consul from 1799 to 1804. As Napoleon I, he was Emperor of the French from 1804 until 1814 and again in 1815. Napoleon dominated European and global affairs for a decade while leading France against a series of coalitions in the Napoleonic Wars. He won most of these wars and battles, building a large empire that ruled over continental Europe before its collapse in 1815. He was one of the greatest military commanders in history, and his wars and campaigns are studied in military schools worldwide. Napoleon's political and cultural legacy has endured, and he has been one of the most celebrated and controversial leaders in world history.Napoleon was born on the island of Corsica not long after its annexation by the Kingdom of France. He supported the French Revolution in 1789 while serving in the French army, and tried to spread its ideals to his native Corsica. He rose rapidly in the Army after he saved the governing French Directory by firing on royalist insurgents. In 1796, he began a military campaign against the Austrians and their Italian allies, scoring decisive victories and becoming a national hero. Two years later, he led a military expedition to Egypt that served as a springboard to political power. He engineered a coup in November 1799 and became First Consul of the Republic. Differences with the British meant that the French faced the War of the Third Coalition by 1805. Napoleon shattered this coalition with victories in the Ulm Campaign, and at the Battle of Austerlitz, which led to the dissolving of the Holy Roman Empire. In 1806, the Fourth Coalition took up arms against him because Prussia became worried about growing French influence on the continent. Napoleon knocked out Prussia at the battles of Jena and Auerstedt, marched the Grande Armée into Eastern Europe, annihilating the Russians in June 1807 at Friedland, and forcing the defeated nations of the Fourth Coalition to accept the Treaties of Tilsit. Two years later, the Austrians challenged the French again during the War of the Fifth Coalition, but Napoleon solidified his grip over Europe after triumphing at the Battle of Wagram.

Hoping to extend the Continental System, his embargo against Britain, Napoleon invaded the Iberian Peninsula and declared his brother Joseph King of Spain in 1808. The Spanish and the Portuguese revolted in the Peninsular War, culminating in defeat for Napoleon's marshals. Napoleon launched an invasion of Russia in the summer of 1812. The resulting campaign witnessed the catastrophic retreat of Napoleon's Grande Armée. In 1813, Prussia and Austria joined Russian forces in a Sixth Coalition against France. A chaotic military campaign resulted in a large coalition army defeating Napoleon at the Battle of Leipzig in October 1813. The coalition invaded France and captured Paris, forcing Napoleon to abdicate in April 1814. He was exiled to the island of Elba, between Corsica and Italy. In France, the Bourbons were restored to power. However, Napoleon escaped Elba in February 1815 and took control of France. The Allies responded by forming a Seventh Coalition, which defeated Napoleon at the Battle of Waterloo in June 1815. The British exiled him to the remote island of Saint Helena in the Atlantic, where he died in 1821 at the age of 51. Napoleon had an extensive impact on the modern world, bringing liberal reforms to the many countries he conquered, especially the Low Countries, Switzerland, and parts of modern Italy and Germany. He implemented liberal policies in France and Western Europe.