The War of the Fifth Coalition was fought between the First French Empire and an Austrian-led coalition from April to October 1809 amid the Napoleonic Wars. The war resulted in Austria's loss of her Mediterranean ports and 20% of her population as a result of territorial changes.
Austria had been forced to make peace with France following the Battle of Austerlitz and the French occupation of Vienna in December 1805, and France went on to defeat Prussia and Russia in the War of the Fourth Coalition in 1807. While this left Napoleon at the peak of his power, his decision to invade Spain in 1808 led to France being mired in the Peninsular War with Spain, Portugal, and Britain, tying down much of its army. France's betrayal of Spain persuaded many in Austria that Napoleon could not be trusted, and Emperor Franz I secretly approached Russia and Prussia for support. As France redeployed 108,000 troops from Germany to the Iberian Peninsula in October 1808, Franz I decided to declare war on France, and - in exchange for British support in the Low Countries and Spain - Austria joined a Fifth Coalition in February 1809. Prussia and Russia ultimately decided to remain neutral, leaving Austria as the sole opponent of France in Central Europe.
Austria's army of regulars, Landwehr home defense forces, and volunteer jaeger units was led by Archduke Charles, while the French army consisted of both veteran soldiers and new conscripts. Napoleon planned to mass 140,000 troops near Regensburg far to the north of where the Austrians were planning to make their attack, and Napoleon began to execute this deployment in April. Austria struck first at the Battle of Abensberg, but the French defeated the central Austrian V Corps on 20 April. The French also held their ground at the 21 April Battle of Teugen-Hausen, and, on 23 April, the French stormed Ratisbon. The Austrians withdrew to Bohemia as Napoleon advanced on Vienna, and the French scored a costly victory at the Battle of Ebersberg on 3 May. Napoleon captured Vienna on 13 May, but Archduke Charles massed his army at the Marchfeld plain northeast of Vienna. On 21 May, he attacked the French as they crossed the Danube, resulting in the Battle of Aspern-Essling. Charles inflicted Napoleon's first major defeat and killed his marshal Jean Lannes, but his exhausted army was unable to pursue the French. Napoleon made another attempt to cross the Danube in July, resulting in the Battle of Wagram. The French scored a costly victory, and the Austrians were forced to sign an armistice on 12 July. Elsewhere, the French routed the Austrians at the Battle of Piave River on 8 May 1809 and invaded Dalmatia. The British attempted to intervene in the war in July 1809 with their Walcheren Campaign in the Netherlands, but fever devastated the British invaders, and they were withdrawn from September to December 1809. Austria also failed to defeat the Duchy of Warsaw in the Austro-Polish War, resulting in Poland's acquisition of Western Galicia. The French also suppressed rebellions in Veneto, Tyrol, and Slovenia, killing the rebel leaders Andreas Hofer and Johann Erker.
The 14 October 1809 Treaty of Schonbrunn saw Austria cede Carinthia, Carniola, and the Adriatic ports to France, Western Galicia to Warsaw, and Salzburg to Bavaria. Napoleon married Marie Louise of Parma, Franz I's daughter, to cement a Franco-Austrian alliance and provide legitimacy to his regime. However, the revolts in Tyrol and Germany worried the French, and German nationalism would once again rally during the War of the Sixth Coalition in 1813.