The siege of Vienna of 10-13 May 1809 saw the Austrian capital fall to Napoleon for the second time in four years after a very short attempt to defend the city.
Vienna was exposed to French attack as a result of the failure of the Austrian invasion of Bavaria that had opened the Franco-Austrian War of 1809. The main Austrian army, under the command of Archduke Charles, had advanced into Bavaria, but had been split in two by Napoleon at the battle of Abensberg (20 April 1809). After the battle of Eggmuhl (22 April 1809) Charles and the larger part of the army had been forced north of the Danube, leaving Hiller and the Austrian left isolated on the south bank. Over the next two weeks Hiller was forced steadily east, before eventually crossing the Danube on 8 May.
The defenders of Vienna were commanded by Archduke Maximilian, the Emperor Franz's cousin. In theory he commanded 34,400 infantry and cavalry and 1,200 artillerymen by the time the French reached Vienna, but most of his troops were inexperienced, exhausted or unreliable. Around 10,000 men were detached from Hiller's corps when it crossed the Danube, but Hiller made sure that most of these men were either inexperienced Landwehr, or new recruits, mostly from Galicia and thus Poles who were hostile to the Austrian monarchy. This division arrived in Vienna on 9 May. Hiller was also forced to sent six battalions of Vienna Volunteers, much against his will as they had performed well during the battle of Ebelsberg (3 May 1809). These troops reached Vienna on 8 and 9 May. 9 May also saw the arrival of Nordmann's brigade with 500 Grenzers and 200 Hussars. Maximilian also had 8,000 Lower Austrian Landwehr at his disposal, and detained two battalions of regular infantry from Lower Austrian as they passed through the city escorting prisoners.
The mood in the city was variable, swinging from determination to panic on a regular basis. Maximilian's best hope of success was to hold out until 19 May, when Charles and the main army was expected to arrive opposite the city, but this depended on Napoleon delaying his arrival for several days. In fact Napoleon was only delayed while he attempted to locate Hiller's retreating army. When he discovered that Hiller had crossed the Danube and partially destroyed the bridge at Krems Napoleon ordered his men to advance towards Vienna, and by the night of 9 May the nearest French troops were at Purkersdorf, only ten miles from the city.
The first French troops to reach Vienna on the morning of 10 May were Colbert's light cavalry. They were followed by Tharreau's infantry from Lannes's corps, and then by Lannes himself. He assumed that the Austrians were not defending the city, and advanced dangerously close to the walls. A burst of Austrian gun fire soon made it clear that the city was still held against the French, while Tharreau was injured by a citizen armed with a wooden plank then had to be rescued from a group of hostile women.
The rest of Lannes's corps, along with Bessières's cavalry, surrounded Vienna during 10 May. Napoleon arrived during the morning, and occupied the Schönbrunn palace (outside the city walls). Late in the day he sent a letter to Maximilian promising to be lenient if the city surrendered, but to destroy it by bombardment if it resisted.
Maximilian didn't respond until the following morning, by which time Hiller had arrived on the opposite bank of the Danube. Kienmayer's II Reserve Corps (just over 4,000 men) was sent into the city before Hiller received an order from Archduke Charles forbidding him from posting men in the city, but these reinforcements encouraged Maximilian, who turned down Napoleon's demand for surrender. Napoleon responded in two ways, first by preparing twenty howitzers to bombard the city, and second by sending Massena's newly arrived 4th Corps to occupy the Prater Island, which sits between Vienna and the main branch of the Danube. That afternoon Massena captured Lusthaus, at the downstream tip of the island, and held it against an Austrian counterattack launched at about 9pm.
This counterattack began at the same time as Napoleon started his bombardment. The twenty howitzers inflicted minimal damage, but caused maximum panic. Chaos enveloped the city. In response Maximilian called a council of war, which met at 1.30am on 12 May. The council decided that the city could not be held, but for the moment Maximilian held out, and instead ordered FML d'Aspre to make a second attempt to dislodge the French troops at Lusthaus. This burst of resolve only lasted for a short time before the Archduke changed his mind and decided to evacuate every regular soldier from the city.
This evacuation took place between 3.30am and 6.30am on the morning of 12 May. FML Andreas Graf O'Reilly was left by default in command of the city, having not received the order to retreat until after the Tabor bridge over the Danube had been destroyed. It was clear that the city would soon be forced to surrender. Once more of Massena's men had reached him, he began to advance up the Prater Island, eventually coming up to the Leopoldstadt suburb, part of the city that had spread onto the island. With the city now totally surrounded O'Reilly began serious surrender negotiations, and the surrender agreement was signed at 2am on 13 May. Later than morning Oudinet's troops entered Vienna, exactly one month after Napoleon had left Paris.
The fall of Vienna didn't bring Napoleon any nearer to victory. The main Austrian army was still intact, and he was faced with the problem of crossing the un-bridged Danube. Napoleon's first serious attempt to cross the river ended with his first serious battlefield defeat, at Aspern-Essling (21-22 May 1809), and even the successful crossing, at Wagram on 5-6 July, didn't produce the sort of crushing victory that he was looking for.