The battle of Linz (17 May 1809) was an unsuccessful Austrian attempt to threaten Napoleon's long lines of communication back from Vienna along the Danube, and to prevent French reinforcements from moving west to join Napoleon's main army.
As the main armies had moved east towards Vienna and the titanic struggles at Aspern-Essling (21-22 May 1809) and Wagram (5-6 July 1809) some sizable detachments had been left behind. On the Austrian side Kolowrat's III corps had been given the task of defending Bohemia. On the French side Vandamme's 8th Corps, made up of troops from Württemberg, had been left to defend a bridgehead across the Danube at Linz, while Bernadotte's Saxon 9th Corps had been left further afield, only beginning their march from Saxony towards Bavaria on 15-16 April. At first Bernadotte's task had been to threaten the Bohemian border, but at the end of the first week of May Bernadotte received orders to move his army to Passau on the Danube, from where it would advance into Austrian to join the left flank of Napoleon's main army. As a result by 16 May Bernadotte's leading troops were only 15 miles from Linz, close enough to take part in the battle on 16 May.
On 7 May Kolowrat received ordered from Archduke Charles to move towards Linz and threaten the French bridgehead. These were followed by fresh orders on 11 May to attack the bridgehead, destroy it then cross to the south bank of the Danube and attack any French reinforcements coming from the west. In theory Kolowrat had enough men to achieve this objective, with 18,234 infantry, including 12,000 regulars, to face Vandamme's 6,830 infantry. Unfortunately for his men Kolowrat had not yet learnt the lessons of Revolutionary and Napoleonic warfare. Instead of concentrating as many men as possible at the decisive point he decided to split his army into three columns, each of which would operate on different lines, and one of which would play no part in the battle. Only half of his infantry would be available on 17 May.
Kolowrat's plan was for his main column to attack along the road from Gallneukirchen, approaching the bridgehead from the east. The right-hand column (Somariva) would attack from the Pöstlingberg, a hill to the north-west of the bridgehead, although large detachments were left behind and patrols sent out to the west of the battlefield. The left-hand column (Saint Julien) was to reach the Danube further east, at Mauthausen, and attempt to cause a distraction. The main result of this plan was to eliminate the left and right hand Austrian columns as effective elements in the battle - Saint Julien played no part whatsoever, while Somariva arrived too late.
The Austrian plan would have had more chance of success if Vandamme hadn't been warned of their approach by a cavalry skirmish at Leonfelden, to the north of the bridgehead, on 16 May. This gave him time to withdraw most of his outlying detachments, and send messengers to Bernadotte asking for help. Vandamme's bridgehead was based around Urfahr, on the opposite bank of the Danube from Linz. From here one road headed east through the village of Katzbach towards Gallneukirchen and the main Austrian attack, while another went north, up the Hellmonsödt valley. Vandamme deployed his light infantry to watch the approaches to Urfahr, with some at Katzbach, some at the mouth of the Hellmonsödt valley and some further up the valley.
The biggest problem with the Austrian plan was that it took the two columns heading for the bridgehead far too long to arrive. The main column was in place by 2pm, Somariva not for another five hours. This meant that Vandamme could deal with each threat in turn. It also meant that the first of the Saxon troops arrived on the battlefield in time to allow Vandamme to use all of his troops in a counterattack.
The main Austrian column, commanded by Vukassovich, but with Kolowrat present, delayed its attack for some time after arriving east of Katzbach. The main attack only began at around 2.30, by which time Kolowrat believed that Somariva was approaching the battlefield. The Austrians captured the villages of Katzbach and Dornach, reached the edge of Steeg (the last village before Urfahr) and also captured the church of St. Magdalena, on a hill at the mouth of the Hellmonsödt valley.
The Austrians were now in a position to threaten the bridgehead, but Kolowrat could see that the first Saxon troops had now arrived, and decided to take up a defensive position and wait for the inevitable French counterattack. This was not long in coming. Vandamme launched his two infantry brigades against the Austrian centre and his light infantry attacked their right. By 6pm the Austrians had been pushed out of St. Magdalena and Dornach. The French advance was held up by some Austrian guns, but when these were captured (partly by Saxon cavalry), Kolowrat decided to retreat, and his main column began to pull back towards Gallneukirchen.
At this point (around 7pm) Somariva finally made his appearance on top of the Pöstlingberg, catching Vandamme and Bernadotte completely by surprise. A determined attack on his part might had turned things around, potentially trapping Vandamme's men between the two Austrian columns, but Somariva could see that his commander was already retreating, and so after firing a few artillery rounds at the bridgehead he prepared to retreat. His sluggish actions gave the French time to respond. Bernadotte personally led two attacks up the hill, but his Saxon troops were inexperienced skirmishers and made little progress. Vandamme responded by ordering his light troops to make their own attack. By this point the Austrians had set up camp on top of the hill, and were caught almost entirely by surprise by this daring night time assault. The Württembergers took 400 prisoners and forced Somariva's much larger column to abandon its camp and retreat into the night.
Both sides now prepared for a renewed offensive, but events elsewhere would soon intervene. Napoleon made his first attempt to cross the Danube at Vienna, and on 21-22 May 1809 suffered his first major battlefield defeat, at Aspern-Essling. In the aftermath of this battle both Kolowrat and Bernadotte were called east to Vienna, and both would lead their corps during the battle of Wagram (5-6 July 1809).