Battle of Abensberg, (20 April 1809)

Napoleon's opening advance in his counterattack against the Austrian army under Archduke Charles in Bavaria in April 1809. Based at Abensberg 25 kilometers southwest of Ratisbon, Napoleon directed two forces, a provisional corps under Marshal Jean Lannes toward Rohr and a Rheinbund force (that is, a force from the Confederation of the Rhine) under Marshal François Lefebvre toward Schweinbach, striking the weakest point in the Austrian line and splitting it in two. The Austrian left wing led by Feldmarschalleutnant Johann Hiller withdrew on Landshut, leaving the main force under Charles south of Regensburg. Napoleon headed for Landshut, while Marshal Louis Davout turned east to engage Charles.
The main Austrian advance (III, IV, and I Reserve Korps) into Bavaria had stalled at Teugn-Hausen on 19 April with three Korps extended southward to protect the western flank. That evening, Napoleon planned to counterattack across a wide front with the intention of enveloping his opponent's army. Assuming that the Austrian army would retreat southward, pressured by Davout, the French emperor formed a provisional corps under Lannes (elements of III Corps and the French reserve light and heavy cavalry), which was sent east from Abensberg toward Rohr to cut the Austrian line of communications, while a combined Bavarian-Württemburg force would intercept any forces reaching the Grosse Laaber river crossings on the road south to Landshut, toward where Marshal André Masséna with IV Corps was marching.
In the late morning, Lannes approached Bachl, forcing Generalmajor Joseph Freiherr von Pfanzelter's small flank force from the Austrian III Korps eastward. The 1st and 3rd Bavarian Divisions with Joseph-Laurent Demont's reserve division headed for Offenstetten and around 10:00 A.M., defeated Generalmajor Ludwig Ritter von Thierry's small detachment, forcing him back on Bachl as Lannes approached from the north. Thierry hastily withdrew to Rohr, which he reached at 2:00 P.M., but its defenders, Feldmarschalleutnant Vincenz Freiherr von Schustekh-Herve's small contingent from V Korps, could do nothing to halt Lannes. The French provisional corps chased the Austrian troops south to the Grosse Laaber crossing at Rottenburg, where they engaged Hiller's VI Korps from 4:30 P.M. until nightfall but failed to cross the river. Meanwhile, the Württemberg contingent and the Bavarian 2nd Division directed by Lefebvre and reinforced by General Jean-Victor Baron Tharreau's cavalry from Marshal Nicolas Oudinot's II Corps had attacked the right flank of Archduke Ludwig's V Korps on the Abens river crossing at Biburg. Nevertheless, Generalmajor Vincenz Freiherr von Bianchi's brigade held them off until ordered to withdraw at 2:00 P.M. The V and II Reserve Korps were pulled back eastward to guard the other Laaber crossing at Pfeffenhausen.
Napoleon's maneuver had split the main Austrian army to the north under Charles (five Korps, including I and II Korps north of Regensburg) from the three Korps (V, VI and II Reserve) to the southwest, forcing Hiller to lead them back on Landshut in some disorder early on 21 April. Napoleon wrongly believed that he had attacked the main Austrian force that day and pursued Hiller's troops, leaving Davout with III Corps to engage Charles.
References and further reading Arnold, James. 1990. Crisis on the Danube: Napoleon's Austrian Campaign of 1809. London: Arms and Armour. Castle, Ian. 1998. Eggmühl 1809: Storm over Bavaria. Oxford: Osprey.