The Battle of Abensberg took place on 20 April 1809 between a Franco-German force under the command of Emperor Napoleon I of France and a reinforced Austrian corps led by Feldmarschall-Leutnant Archduke Louis of Austria.

As the day wore on, Feldmarschall-Leutnant Johann von Hiller arrived with reinforcements to take command of the three corps that formed the Austrian left wing.

After Marshal Louis-Nicolas Davout's hard-fought victory at Battle of Teugen-Hausen the previous day, Napoleon determined to break through the Austrian defenses behind the Abens River.

While the Austrians initially held the river line, Lannes' strike force crashed through Louis' defenses farther east.

On the left, the Austrians managed to conduct a capable rear guard action, but during the day the French smashed their opponents' right flank and captured thousands of soldiers.

The next day, Hiller withdrew to Landshut, separating the left wing from the main army under Generalissimo Archduke Charles, Duke of Teschen near Regensburg (Ratisbon).

The French surrender of Regensburg on 20 April allowed Charles' army a retreat route to the north bank of the Danube.

[4] Napoleon's deputy commander Marshal Louis Alexandre Berthier mismanaged the Grande Armée's concentration, leaving its units scattered across a broad front in a confused state.

[5] The central mass of Archduke Charles' 209,600-man host[6] crossed the Isar at Landshut on 16 April, but the next day Emperor Napoleon arrived at the front from Paris.

[8] In the center, General of Division Louis-Pierre Montbrun's 3,800 cavalry and infantry skilfully held off Feldmarschall-Leutnant Prince Franz Seraph of Rosenberg-Orsini's much stronger IV Armeekorps for most of the day.

Ultimately, Davout brought superior forces to bear in the late afternoon and pushed back the Austrians a short ways.

As an additional link between Thierry and III Armeekorps, Hohenzollern detached General-Major Joseph Freiherr von Pfanzelter with a Grenz infantry battalion and two squadrons of hussars, about 1,000 men.

While successfully defending the city, Colonel Louis Coutard's 2,000-man 65th Line Infantry Regiment ran dangerously low on small-arms ammunition.

[21] Hiller also posted General-Major Armand von Nordmann with a small force at Moosburg an der Isar to watch his south flank.

Napoleon ordered Masséna's IV Corps to capture Landshut and the Isar River crossings, cutting the Austrian line of communications.

[26] Schustekh's infantry had just rejoined V Armeekorps after marching from Mainburg with General-Major Joseph, Baron von Mesko de Felsö-Kubiny's detachment.

[27] In one account, Lannes' advance guard approached Bachl in the late morning of 20 April, forcing Pfanzelter's small detachment from the Austrian III Armeekorps eastward.

Eventually, Jacquinot got his entire brigade into action and pressed the Austrian hussars back on the supporting battalion and one-third of Grenz infantry south of Rohr.

With the help of Gudin's 17th Light Infantry and a battery of artillery, Jacquinot's chasseurs broke Thierry's foot soldiers and hounded them into the woods again.

Though a road ran directly from Mainburg to Louis' left flank at Siegenburg, a distance of only 13 kilometers, Hiller elected to join his colleague by a roundabout march via Pfeffenhausen.

Hearing troubling reports from the right flank, he sent Feldmarschall-Leutnant Karl von Vincent toward Rottenburg with the line brigades of Generals-Major Josef Hoffmeister and Nikolaus Weissenwolf, plus four squadrons of the Rosenberg Chevauxlegers Regiment # 6.

[34] Archduke Louis posted Prince Reuss and General-Major Joseph Radetzky von Radetz with four battalions and 12 cavalry squadrons at Siegenburg.

Lieutenant-General Karl Philipp von Wrede probed at Siegenburg but his 2nd Bavarian Division was easily fended off by Radetzky and a battery of 12-pdr cannon.

At this time, Louis heard that the French attacked Thierry, so he ordered Radetzky to send two battalions to the right flank and called up Kienmayer's command from Ludmannsdorf.

[39] Pressed by Wrede's 7th Bavarian Infantry Regiment, plus two of Hügel's Württemberg battalions,[37] Radetzky conducted an orderly retreat on the Siegenburg road, covered by the grenzers and Kienmayer's grenadiers under General-Major Konstantin Ghilian Karl d'Aspré.

Radetzky fell back to a hill called the Hornbach where his rear guard skirmished with the Bavarians into the early morning hours.

A charge by the Rosenberg Chevauxlegers halted the French cuirassiers long enough for his infantry to gain solid positions on the east side of the Große Laber.

[42] On 20 April, the Austrian II Armeekorps continued attacking Colonel Louis Coutard's 65th Line Infantry Regiment at Regensburg.

[46] When Kollowrat finally reported the II Armeekorps available for duty that evening, the headquarters ordered his troops to make an overnight march west to Hemau on the north bank of the Danube.

This crucial decision meant that the Austrian left wing would in the near future operate independently from Archduke Charles' main body.