In the Battle of Aspern-Essling (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.

The French lost over 20,000 men including one of Napoleon's ablest field commanders and closest friends, Marshal Jean Lannes.

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.

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, Le Febvre'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.

[8] Outnumbering the French Emperor, Francis urged the archduke to take immediate offensive action but Charles had no wish to venture a major attack across the Danube.

He was concerned about the potential for unrest in his rear, the approach of John's army and the possibility of Prussian intervention and a reversal of Russia's position.

Undeterred by the news of heavy attacks on his rear from Tyrol and from Bohemia, Napoleon ferried all available troops to the bridges, and by daybreak on the 21st, 25,000 men in three infantry and two cavalry divisions[7] were collected on the Marchfeld, the broad plain of the left bank which was also to be the scene of the Battle of Wagram.

His forces on the Marchfeld were drawn up in front of the bridges facing north with their left flank in the village of Aspern (Gross-Aspern) and their right in Essling.

[11] Kaiserlich-Königliche Hauptarmee, under the command of Charles of Austria:[13] Total: 99,000 men; 84,000 infantry, 14,250 cavalry, 288 guns The corps led by Johann von Hiller (VI), Heinrich Graf von Bellegarde (I) and Prince Friedrich of Hohenzollern-Hechingen (II) were to converge upon Aspern, while Prince Franz Seraph of Rosenberg-Orsini (IV) was to attack Essling.

Lannes, the official commander of II Corps, was given temporary control of Boudet's division and charged with defending Essling, the most vulnerable part of the French position.

The significant deployment of light cavalry at the head of the crossing schedule suggests that he still anticipated the necessity to locate Charles' army.

Despite the expectation that Bessières' advanced cavalry patrols would have discovered the presence of the main Austrian army so close to the bridgehead, they sent back no such information.

The French were caught off-guard by the unexpected attack, which was disguised by the combination of a low ridge and a convenient dust storm until the last moment.

By exerting considerable effort, he was able to repel the initial Austrian attack and gain sufficient time to bring all four of his regiments into the town.

General Molitor was now facing significant challenges in maintaining his position, with the enemy launching simultaneous attacks from three directions and overwhelming his forces.

However, Molitor was determined not to concede defeat and continued to hold his position until he could be reinforced by General Legrand and subsequently Carra St. Cyr.

St. Germain's brigade from Nansouty's division and St. Sulpice's cuirassiers were present for Bessiéres' final assault on the enemy centre, but there were no significant outcomes.

With the onset of darkness, the majority of combat ceased, and the two armies settled into a state of relative calm, with the exception of the ongoing engagement at Aspern, where a narrow distance separated the French and Austrian forces.

[18] Shortly after 7:00 a.m. Lannes' corps began to advance in echelons, with Saint-Hilaire leading on the right and Oudinot following in the centre and on the left with Claparéde's and Tharreau's divisions.

They marched into a sustained bombardment, but the Austrians at the front wavered, forcing Charles to send his reserve grenadier divisions forward, bringing the French advance to a halt.

This, together with the fact that Lannes failed to bring the Austrian center down, left Napoleon with no alternative but to order a gradual withdrawal to Aspern and Essling.

Subsequently, Rosenberg's IV Corps, supported by Merville's Grenadier Brigade, led by Charles, forced Boudet out of Essling.

In his subsequent bulletin, Napoleon acknowledged 4,100 killed and wounded, characterising the battle of Aspern-Essling as a new memorial to the glory and inflexible firmness of the French army.

He then wrote a letter to his brother: Lieber Herr Bruder, Erzherzog Karl!Ihnen war es vorbehalten, das 15 jährige Waffenglück des stolzen Gegners zuerst zu unterbrechen.

Marcellin Marbot, one of Marshal Lannes aide-de-camps, wrote in his memoirs of the battle, in which he had to observe the last moments of his close friends, and describes the amount of bloodshed and sadness which came to the Grande Armée after the crossing of the Danube.

The army surgeon Dominique-Jean Larrey also described the battle in his memoirs and mentions how he fed the wounded at Lobau with a bouillon of horse meat seasoned with gunpowder.