The Battle of Eckmühl (also known as "Eggmühl") fought on 21 April – 22 April, 1809, was the turning point of the 1809 Campaign, also known as the War of the Fifth Coalition. Napoleon I had been unprepared for the start of hostilities on April 10, 1809, by the Austrians under the Archduke Charles of Austria and for the first time since assuming the French Imperial Crown had been forced to cede the strategic initiative to an opponent. Thanks to the dogged defense waged by the III Corps, commanded by Marshal Davout, and the Bavarian VII Corps, commanded by Marshal Lefebvre, Napoleon was able to defeat the principal Austrian army and wrest the strategic initiative for the remainder of the war.
The French attack, however, had only split the Austrian Army, separating its Left Wing, composed of the V A.K., VI A.K. and II Reserve A.K., from the balance of the army. Two corps, III A.K. and IV A.K., were withdrawn by Archduke Charles to the North, forming a nine-mile line from Abbach on the Danube to Eckmühl on the Gröss Laber. More importantly, unbeknownst to Napoleon, the Austrians gained a victory of their own on April 20, 1809, by surrounding and capturing the French garrison at Ratisbon and its strategic bridge over the Danube. The capture of the bridge at Ratisbon allowed Charles to re-establish contact with his Right Wing, General der Cavallerie Bellegarde's I A.K. and FZM Kollowrat's II A.K., hitherto separated from the rest of the Austrian Army by the Danube.
With the seizure of the bridge at Ratisbon, Archduke Charles no longer needed to defend the Landshut bridgehead and instead moved to concentrate his remaining forces so as to envelop and destroy Davout's corps. FML Hohenzollern's III A.K. (Approx. 15,700 men) and FML Rosenberg's IV A.K. (Approx. 21,460 men), were ordered to hold the Austrian left, pinning in place Davout's corps, while FZM Kolowrat's fresh II A.K. (Approx. 28,168 men) and the elite grenadiers and cuirassiers of G.d.K. Liechtenstein's I Reserve A.K. advanced South from Ratisbon and deployed against Davout's exposed left flank. Inexplicably, no orders were issued to G.d. K. Bellegarde, so his powerful I A.K. (Approx. 27,653 men) remained on the North Bank of the Danube and played no role in the subsequent fighting.
For his part, Napoleon was intent on enveloping and destroying the Austrian forces retiring Southwest to Landshut and its bridge across the Isar. The II and IV Corps (App. 57,000 men under the overall command of Marshal Masséna) were directed to cross the Isar upstream from Landshut and block the Austrians from crossing to the South Bank. Meanwhile, under the overall command of Marshal Lannes, Lannes' Provisional Corps, the VII (Wuttenberg) Corps, a division from VII Corps and two cuirassier divisions (App. 51,000 men) were to closely pursue and destroy the defeated Austrians. The mop-up of what Napoleon thought was a "curtain of three regiments" was left to Davout, even though more than half of the III Corps' original units had been detached to create Lannes' task force. . Despite Davout's reports to the contrary, Napoleon ordered him to attack the Austrians on his front in the morning, with the proviso that Lefebvre's equally depleted corps would support him if he needed help (A total of approximately 36,000 men for both corps).
The vanguard of the assault were the German troops under General Vandamme; these soldiers stormed the bridge at Eckmühl and even captured the town's chateau after ferocious Austrian resistance. At this point, Davout launched his men against the Austrian center at the village of Unterlaichling and the woods to the north. The famous 10th Legere Regiment became involved in vicious fighting around the woods, but eventually was strengthened by Bavarians under Deroy and managed to capture the positions. North of Unterlaichling, Davout's troops under Louis Friant and St. Hilaire steadily pushed back the defenders of Oberlaichling and the surrounding woods, overran a redoubt held by Hungarian grenadiers, and prompted Charles to order a general retreat.
The struggle now devolved into a series of major cavalry clashes as the Austrians attempted to extricate their army without losing too many prisoners. Perhaps the best cavalry in the Habsburg army, the Vincent Chevaulegers and the Stipsic Hussars, occupied the Bettelberg ridgeline between Eckmühl and the woods above Unterlaiching. These elite units demolished some German light cavalry before being stopped by Bavarian infantry. Napoleon was insistent on the immediate capture of this position and ordered forward two heavy cavalry divisions under St. Sulpice and Nansouty. These horsemen were pummeled by Austrian artillery but came on nonetheless and managed to saber the gunners after having seen off the enemy cavalry.
The first phase of the retreat ended, but it was not over yet. The Austrians had found a chokepoint in the road and were instructed to stem the French tide. Three French cuirassier divisions supported by additional German light cavalry attacked and a swirling melee developed. The Austrians fought heroically but were heavily outnumbered and had to retreat. During this part of the conflict, more French cavalry struck in their flank and the remaining Austrian horse fled north to Ratisbon with great celerity.
Nevertheless, the French inflicted 12,000 casualties at the cost of just 6,000, and Napoleon's speedy arrival witnessed an entire axial realignment of his army (from a north-south axis to an east-west one) that permitted the defeat of the Austrians. Subsequent campaigning led to the French recapture of Ratisbon, Austrian eviction from Southern Germany, and the fall of Vienna.