The battle of Ratisbon or Regensburg (23 April 1809) was the final major battle in the initial Bavarian phase of the Franco-Austrian War of 1809 (Fifth Coalition), and saw the French push the Austrians out of their last foot hold on the southern bank of the Danube.
The Austrians held the initiative in Bavaria from their initial invasion on 10 April until the end of 19 April. On that day their attempt to trap Davout's corps around Regensburg failed (battle of Teugn-Hausen), and the French army was reunited, after a period when it had been dangerously spread out.
On the following day Napoleon launched his counterattack. The resulting battle of Abensberg (20 April 1809) saw the French split the Austrian army in half, forcing the Austrian left to retreat east towards Landshut. Archduke Charles, with the right wing of the army, remained static around Eggmühl.
On 21 April Napoleon led the bulk of his army after the Austrian left, catching and defeating them at Landshut (21 April 1809), although most of the Austrian troops escaped to the east. On the same day Davout, who had been left to deal with what Napoleon believed was the shattered remains of the Austrian right, ran into the Archduke around Eggmühl. Napoleon was forced to turn north to deal with this new threat. The resulting battle of Eggmühl was another French victory, but the bridge across the Danube at Regensburg had been taken by the Austrians late on 20 April, and so Charles was able to escape north across the Danube, leaving a rearguard to defend the city.
That rearguard was commanded by GM Karl von Fölseis, normally the commander of a brigade in II Corps. He was given two battalions from Infantry Regiment No.15 (Zach) and three from IR.25 (Zedtwitz), as well as a brigade artillery battery. A significant part of the Austrian army had crossed the river by the time the first French cavalry appeared (some time between 8am and 9am), but II Corps and large parts of the III and IV Corps were still on the southern bank. The result was a large cavalry battle outside the city. Napoleon had around 8,000 cavalry at his disposal, the Austrians only 5,000, but the Austrians managed to delay the French advance. The general trend of events also tended to push the French west, away from a crucial pontoon bridge just to the east of Regensburg. Eventually the pressure of numbers forced the Austrians to retreat back into Regensburg, and the last open gate was slammed shut before the French could get into the city.
This left Napoleon with a major problem. Regensburg was not strongly fortified by contemporary standards, but it was still a fairly strong position, and Napoleon couldn't afford to leave it in Austrian hands. Napoleon had a choice of two main strategies after the fighting in Bavaria - either to cross the Danube and pursue the main Austrian army, or to advance east along the southern bank of the Danube to threaten Vienna. If he chose the first option then the bridge at Regensburg was essential, if he chose the second then it could not be left in Austrian hands, as that would have left the French lines of communication open to attack.
The last gate into Regensburg was closed by noon, and by 1pm the French had surrounded the city. Clearly a formal siege was out of the question, as it would have taken far too much time and given Charles time to rebuild his army. Instead Napoleon chose to launch a series of assaults against the city. The first, using III/7th Léger, 12th Ligne and 85th Ligne, the first infantry units to reach the area, was an attempt to catch the Austrians by surprise, and failed.
Napoleon's next move was to bring up some of his guns and began a bombardment of the south-east corner of the walls, close to Peter's Gate. During this bombardment the Emperor suffered a minor wound when a spent bullet hit his foot. Although the bullet failed to break his skin, news of the wound caused some nervousness across the army, forcing Napoleon to spend some time visiting his troops.
At about 3pm part of a tower and the outer wall of a house finally collapsed, partially falling into the dry ditch outside the walls. The resulting breach was still far too small for a conventional assault, but Napoleon was unwilling to wait. A second assault was launched, with no more success than the first. A third attack also failed. By this time the supply of volunteers was running short. Marshal Lannes, who was in command of the attack, attempted to convince his men to make a fourth attack (third on the breach), but it took a dramatic gesture to achieve anything. Frustrated with the failure of his men to respond, Lannes picked up a ladder and prepared to lead the next attack himself. His staff officers attempted to talk him out of it, volunteering to lead the attack themselves. This public argument inspired (or shamed) his men. The next attack was led by Jean Baptiste Marbot and Charles François Huchet de la Bédoyère, two of Lannes's staff officers, and involved men from the 25th and 85th Ligne. Marbot would late claim to be the first officer to reach the top of the walls, followed by de la Bédoyère.
This attack finally managed to break through the breach. The French then reached a postern gate, which allowed them to let reinforcements into the city. These troops captured Peter's Gate from the inside, and by 4pm Gudin's division was inside the walls. The fighting then developed into two separate battles. The French quickly reached the southern end of the stone bridge across the Danube, and for the next five hours were involved in a fight with the Austrian troops defending the fortified bridge, before eventually the French managed to gain a toehold on the north bank, capturing the suburb of Stadtamhof. The second part of the battle was a vicious street fight that lasted for about the same time, and mixed in with an outbreak of looting to produce a hellish result. The eventual Austrian casualties of around 8,900 killed, wounded and missing included 3,700 men from the garrison of Regensburg.
The fall of Regensburg ended the Bavarian phase of the war. In just under a week Napoleon had transformed the situation, turning a potential disaster into a stunning victory (if not quite as stunning as he would have hoped). The main Austrian army had been split in two, Archduke Charles had been thrown off the southern side of the Danube, and the road into Austria was now open.