In the Battle of AspernEssling (21–22 May 1809), Napoleon crossed the Danube near Vienna, but the French and their allies were attacked and forced back across the river by the Austrians under Archduke Charles. It was the first time Napoleon had been personally defeated in a major battle, as well as his first battle defeat in 10 years since the siege of Acre, and his first battle defeat as head of state, although he did suffer a tactical defeat in the Battle of Caldiero and in the Second Battle of Bassano. Archduke Charles drove out the French but fell short of destroying their army. The French lost over 20,000 men, including one of Napoleon's ablest field commanders and closest friends, Marshal Jean Lannes.
Background
On 10 April 1809, Austrian troops under Archduke Charles had crossed the border into Bavaria, a French client state; thus beginning hostilities without a declaration of war. The French, initially taken by surprise, were able to defeat the Austrians at Eckmühl, Abensberg and Ratisbon. The Archduke, fearing that his army might be destroyed, retreated towards Bohemia. After these victories the way to Vienna for Napoleon was eventually open. By 13 May Napoleon had entered the City.
Prelude
Considering the French army would need to rest at Vienna, Charles decided to march there, joining Hiller at the Marchfeld. When he arrived there, he had an army of 115,000 men at his disposal. Napoleon could muster only 82,000 troops. The 8th and 9th Corps, numbering 38,000, were back on the River Traun. They were observing Johann Kollowrath's 25,000 Austrian troops who were entrenched on the north bank of the Danube near Linz. At the same time, they were safeguarding Napoleon's communications with France and his current centre of operations. They were also dealing with the inconvenience posed by local risings, which were becoming a constant source of concern. Davout, having previously crossed his corps to the south bank of the Danube via Straubing and Enns, was engaged on similar duties with 35,000 men near St. Polten on the River Traisen, approximately 40 miles west of Vienna. Meanwhile, Lefebvre's 7th Corps, comprising 22,000 personnel, was engaged in monitoring the movements of Archduke John's forces, specifically 8,000 troops stationed in Innsbruck and 7,000 more under Jellacic on the higher reaches of the River Enns, from the vicinity of Salzburg. In addition, the remaining 30,000 Austrians of Archduke John's army was responsible for maintaining a watch on the 57,000 Frenchmen of Eugene’s Army of Italy in Carinthia and Carniola.
Francis urged the Archduke to take immediate offensive action, given numerical superiority, but Charles had no wish to venture a major attack across the Danube. Charles argued that, by remaining on the left bank of the river, the army would retained freedom of manoeuvre. Napoleon wanted an early battle