The War of the Third Coalition was the first of the Napoleonic Wars, fought from 1805 to 1806 between the First French Empire and a coalition of Britain, Austria, Russia, Naples, Sicily, and Sweden.
In March 1802, France and Britain had agreed to cease hostilities under the Treaty of Amiens, restoring peace in Europe for the first time in 10 years. However, British troops violated the treaty by remaining on Malta, while Napoleon attempted to reconquer Haiti in the Caribbean, resulting in continued tensions and the resumption of war on 18 May 1803. After Napoleon's expeditionary army in Haiti was destroyed by disease, Napoleon abandoned his plans to rebuild his New World empire and sold the Louisiana Territory to the United States on 30 April 1803. He spent all 60 million francs ($15 million) on building an army for an invasion of England, readying a Grande Armee at Boulogne for the assault. However, Britain signed an agreement with Sweden in December 1804, putting Swedish Pomerania's naval bases at the disposal of the Royal Navy. The French execution of the Royalist plotter Louis Antoine, Duke of Enghien shocked the aristocrats of Europe, who feared the continuation of the French Revolution. From 1804 to 1805, British prime minister William Pitt the Younger built a "Third Coalition" against France. On 11 April 1805, Britain allied with the Russian Empire, and Austria, Sweden, and Naples eventually joined the alliance. Meanwhile, Napoleon formed the First French Empire in May 1804 and was crowned Emperor on 2 December 1804. Napoleon proceeded to create eighteen Marshals of the Empire in preparation for war.
By 1805, Napoleon's Grande Armee had grown to 350,000 troops. At the same time, the Imperial Russian Army suffered from outdated organization, while the Austrian Reichsarmee was undergoing reforms under the leadership of Archduke Charles. In August 1805, Napoleon turned his attention from the English Channel to the Rhine to deal with the new Austrian and Russian threats, cancelling his invasion of the British Isles.
The Austrian general Karl Mack von Leiberich was dispatched to seal off the Black Forest to contain the French until the Russian general Mikhail Kutuzov could join him at Ulm, while Austria focused its efforts on northern Italy and sent Archduke Charles' 95,000-strong army to attack Mantua, Peschiera, and Milan. Napoleon responded by sending Joachim Murat to make demonstrations across the Black Forest as other forces would swing around Mack, capture Augsburg, and isolate the Austrian army. On 8 October 1805, the first action occurred at the Battle of Wertingen, and the French victory there enabled the French to begin their encirclement of Mack's army. After the 14 October Battle of Elchingen, Mack's army was surrounded in Ulm, and he surrendered on 20 October as the French began to bombard the city. A day later, however, the British Royal Navy achieved naval supremacy by defeating a Franco-Spanish fleet at the Battle of Trafalgar off Spain.
Following the victory at Ulm, the Grande Armee followed the retreating Austrians towards Vienna. On 5 November, the Russians and Austrians fought a rearguard action at the Battle of Amstetten. Several other delaying actions would be fought, including the Battle of Durenstein and the Battle of Schongrabern, and, on 1 December, the French and the Austro-Russian armies met at the Battle of Austerlitz east of Brno in Moravia. The French attacked the Allied army and destroyed its isolated sections, and the Allied army abandoned the field in a panic.
In Italy, the French defeated the superior Austrian army at the Battle of Caldiero on 29-31 October, after which they blockaded Venice. The French also invaded the Kingdom of Naples, and Naples' Russian defenders evacuated after Austerlitz. The British forces in Naples likewise withdrew to Sicily, and Andre Massena's French I Corps invaded Naples on 9 February 1806. King Ferdinand IV of Naples fled to Sicily on British ships, vainly counting on the people of Naples to rise up against the French as they had done in 1799. The Neapolitan army in Calabria was crushed at the Battle of Campo Tenese on 10 March 1806, and Joseph Bonaparte was crowned King of Naples a day later. On 18 July 1806, the last Neapolitan stronghold of Gaeta surrendered to the French. The British had attempted to relieve the siege of Gaeta, winning the 4 July 1806 Battle of Maida but failing to rescue Naples. A Calabrian insurrection did break out, and a brutal guerrilla war continued until 1807.
In just three months, France was able to occupy Vienna, decimate two armies, and humble the Austrian Empire. On 4 December, Austria signed a truce with France, exiting the war 22 days later and ceding Veneto to the Kingdom of Italy. Russia was allowed to withdraw its army, while the French encamped themselves in southern Germany and created the Confederation of the Rhine in July 1806. British and Sicilian troops would guard the Bourbon monarchy on Sicily until 1815, defeating Joachim Murat's many failed invasions. As Russia, Britain, and other powers remained at war with France, no peace followed Austria's exit from the war, and Prussia would join a "Fourth Coalition" in 1806.