The Situation at the end of 19 April
The battle of Abensberg (20 April 1809) was the first stage in Napoleon's counter-attack against the Austrian army invading Bavaria at the start of the Franco-Austrian War of 1809, and saw him split the main Austrian army in half, forcing it to retreat to separate directions.
At the start of the Austrian campaign on 10 April the French had been in a rather vulnerable position. Marshal Berthier, the command of the French Army of Germany, mismanaged his troops, eventually producing a situation where Marshal Davout's corps was isolated north of the Danube close to Regensburg, while the rest of the army was some way to the south-west and on the opposite side of the river.
Napoleon arrived to take personal command on 17 April, and immediately ordered Davout to cross the Danube and march south-west along the Danube to rejoin the rest of the army. At about the same time Archduke Charles, commander in chief of the Austrian army, had realised that he had a chance to defeat Davout in isolation. On 19 April the Austrians attempted to move north/ north-east to get between Davout and the rest of the French army, but Charles had slightly misjudged the situation. His plans were based on the idea that Davout was going to remain at Regensburg, when in fact he was already on the move. Two of Charles' three corps thus missed the French, and only his left hand corps was involved in the battle of Teugn-Hausen. Davout was able to hold off the Austrian attack, and his entire corps was able to escape from the trap. The French army was now largely united, and Napoleon was ready to begin his counter attack.
The Situation at the end of 19 April
After the events of 19 April the Austrian army was already divided into two wings. Archduke Charles was in direct command of the right wing of the army south of the Danube. This was stretched out from west to east. III Corps (Hohenzollern) was still around Hausen, where it had fought that day. IV Corps (Rosenberg) was four miles to the east, at Dünzling. Finally I Reserve Corps (Liechtenstein) was another four-five miles to the east, on the road north from Eggmühl.
The left wing of the army, under the command of Hiller, was some way to the south west. V Corps (Ludwig) and II Reserve Corps (Kienmayer) were east of the River Abens, centred on Siegenburg, around 20 miles to the south-west of Hausen. Hiller was another eight-ten miles to the south, at Mainburg.
Another two corps were north of the Danube, with one just north of Regensburg and a second around Amberg.
Napoleon was generally aware of the location of most of the Austrian corps, but not of the two corps at the right of their line, at Dünzling and north of Eggmühl.
The French were also rather spread out. 3rd Corps (Davout) was on the left, centred on Teugn. To his right Lannes, who only reached the front after serving in Spain late on 19 April) was given command of a strong provisional corps, which would be the main attacking force on 20 April. Next in line was Lefebvre's 7th Corps, around Abensberg, with 8th Corps (Vandamme) close behind. A small garrison still held Regensburg, with its crucial bridge across the Danube. Finally a strong force was further to the right - Massena's 4th Corps and Oudinot's 2nd Corps were around Pfaffenhofen, twenty five miles to the south-west of Abensberg.
Napoleon plans for 20 April aimed at the destruction of the Austrian army. The main thrust would come from Lannes, Lefebrve and Vandamme. They were to attack the Austrian centre, break through it, and then split, with part of the main force heading towards Landshut to cut off any Austrians trying to retreat east to the south of the Danube, and the rest turning left in an attempt to trap the rest of the Austrian army against the Danube. The bridge at Regensburg was the key to this plan - as long as it remained in French hands the Austrian right wing had no way to cross the Danube, but unknown to Napoleon Regensburg was only held by a small garrison.
Further south Massena was ordered to march east, with the ultimate objective of reaching Landshut, where he would cut off one major Austrian line of retreat. Another part of his force was to turn north to join the troops at Abensberg.
On the French left Davout's role was to pin down what Napoleon believed to be the shattered remnants of the Austrian right wing, crushed on the pervious day at Teugn-Hausen.
Napoleon's plan had two main flaws. The first was based on incorrect intelligence - the Austrian right had not been crushed on 19 April, and so when the Austrian army was split in half Charles still had three complete corps under his command. The second was that Massena was unable to reach Landshut as quickly as expected, allowing the Austrian left to escape across the Lech.
Early on 20 April Lannes began the battle, advancing south-east towards Bachl. To his right the 1st Bavarian Division moved east, pushing an Austrian force (Thierry) out of Offenstetten. Thierry attempted to retreat towards Bachl, but instead ran into French light cavalry. Thierry's brigade began to collapse, and attempted to escape further south-east to Rohr. Lannes followed close behind, and the Austrian defenders of Rohr were quickly forced to retreat east to a new line on the Grosse Laaber River. As the Austrians retreated east, Lannes continued south-east towards the same river at Rottenburg.
At Rottenburg Lannes ran into Hiller's corps, which had advanced up the Grosse Laaber during the course of the day. The clash began in early evening with an Austrian attack across the river, but this was quickly repulsed, and Hiller was forced to stay on the defensive until darkness fell. That night he abandoned his position at Rottenburg, and began the retreat back to Landshut.
The second main French attack didn't begin until around noon. The 2nd Bavarian Division crossed the Abens at Biburg, to the right of Lannes's corps, and attacked the Austrians of Bianchi's brigade. This time the Austrians were able to hold on to their initial positions, and the Bavarians weren't able to make any progress for a couple of hours. By this point Lannes and the 1st and 3rd Bavarian Divisions had advanced a great distance, and so Napoleon decided to commit his Wurttemberg troops to support the 2nd Bavarian Division. These extra troops convinced the Austrians that it was time to retreat, and Bianchi and the nearby Radetzky were ordered back to the Laaber around Pfeffenhausen.
Wrede's 2nd Bavarian Division had one more contribution to make to the battle, when Napoleon ordered them to make a night attack on the new Austrian position at Pfeffenhausen. The unexpected appearance of the Bavarian troops caused a panic in Pfeffenhausen. The Austrians in the town prepared to retreat to Landshut, and in the chaos the Bavarians were able to cross the burning bridge over the Laaber and capture the village.
This ended the day's fighting, with the French on the Laaber, the Austrian left in full retreat and the Austrian right, complete with Archduke Charles, isolated to the north.
At the end of 20 April the main Austrian army had been split in two and was retreating in separate directions. Hiller's wing was forced to move east towards Landshut, after suffering 7,200 casualties on 19-20 April. In contrast the French and their German allies had only suffered around 1,000 casualties on the two days.
The only good result for the Austrians on 20 April was the surrender of the French garrison in Regensburg. The garrison had been holding out against an attack from the north, but when Austrian troops also appeared to the south their position was hopeless. The strong stone bridge over the Danube at Regensburg thus fell intact into Charles's hands, giving him an escape route across the Danube.
The Austrians also benefited from a stroke of luck. Napoleon still didn't know how strong the Austrian right wing actually was, and so on 21 April he focuses all of his attention on the retreating Austrian left, in the belief that this was the main body of their army. As a result Davout was left to face the Archduke's main force around Eggmuhl, while the main French and Allies army advanced towards Landshut, where a battle developed on 21 April.