The combat of Riedau (1 May 1809) was a minor rearguard action fought during the retreat of the left wing of the main Austrian army after the failure of their invasion of Bavaria. The Austrian invasion had progressed too slowly, giving Napoleon time to arrive at the front. On 20 April he had launched his counter-attack, splitting the Austrian army in half during the battle of Abensberg. Further victories had followed at Landshut (21 April) and Eggmuhl (22 April) and the main part of the Austrian army, under the command of Archduke Charles, had been forced to retreat to the northern bank of the Danube. This left FML Johann Freiherr von Hiller isolated on the southern side of the river with the left wing of the army. Hiller had been the main target of Napoleon's pursuit on 21 April at Landshut, but had been saved from further defeats when Napoleon was forced to turn north to save his own left wing from defeat at the hands of the main Austrian army at Eggmuhl. Over the next few days Hiller retreated east back into Austria. He safely crossed the Inn, and continued to pull back, reaching Ried on 28 April, and was on the Traun River by the end of the month.
The fighting on 1 May was a result of Hiller's overoptimistic reading of the situation. The French had reached Burghausen on the Salzach River on 28 April, but the bridge over the river had been destroyed, and so for two days the French were stuck on the Bavarian side of the river, looking across to Austria (the border ran south-west along the Inn, then south along the Salzach towards Salzburg). Hiller knew that the French had some men across the Inn at Schärding, but underestimated their numbers. He was also convinced that the main French force was still on the west bank of the Salzach. On the morning of 1 May he ordered his covering troops to move west in an attempt to find the French.
Unfortunately for those troops the bridge at Burghausen had been repaired on 30 April. Napoleon had crossed the river with around 50,000-60,000 men, and advanced north-east, ending the day at Braunau on the Inn. There was a minor clash between French and Austrian troops at Altheim, on the road east from Braunau, but the Austrians still failed to realise that the French were across the river in such force.
A number of separate engagements broke out between the advancing French and the Austrians on 1 May. To the south a small Austrian column was advancing along the road from Ried towards Altheim when it ran into part of the French advance guard under the command of Oudinot. 1,200 Austrians were captured in an initial clash near Polling, and the troops defending Ried were also forced to pull back. The French advance only ended at Geiersberg, after a ride of 30 miles.
The fighting further north at Riedau involved part of Massena's column. While the main part of his corps advanced along the main road toward Waizenkirchen (and eventually to Linz on the Danube), his right flank was protected by a force made up of the 4th Ligne infantry and the Baden Light Dragoons and Wurttemberg Leib Chevaulegers, all under the command of Adjutant Commandant Anne Alexis Jean Trenqualye.
Trenqualye's men were advancing towards the rearguard of Generalmajor Emmanuel Freiherr von Schustek-Herve's division, which was centred on Neumarkt im Hausruckkreis. A minor road linked Riedau to Neumarkt and then continued to the River Traum. Schustek's rearguard at Riedau consisted of the six infantry companies of III/ Jordis and two squadrons of Kienmayer Hussars. The Austrians were strung out along the road, with cavalry pickets near Riedau, then a line of infantry skirmishers, then two companies of infantry in the centre and finally the remaining four companies of infantry and the cavalry and the eastern end of the line.
Trenqualye's men easily brushed aside the Austrian cavalry pickets, but then ran into the infantry skirmishers. Trenqualye decided to use his Baden dragoons to outflank the Austrian infantry, sending two squadrons to the north and two to the south. At the same time the Wurttemberg cavalry and the voltiguer companies from the 4th Ligne, were sent to attack along the road.
The key to the success of this attack was timing, and it very nearly went wrong. The Wurttemberg force came under heavy fire from the infantry, provoking its commander into launching an attack before the flanking forces were in place. Luckily for him the southern flanking force arrived just in time to take part in this attack. The two forward Austrian infantry companies were attacked from two sides at once and destroyed.
The rest of the Austrian rearguard now attempted to enter the fight. First into action was the Kienmayer Husssars. They were close to success when the northern flanking force of Baden dragoons finally reached the scene. This turned the tide of the cavalry action, and the Hussars turned and fled.
The four remaining companies of III/ Jordis were now dangerously isolated. They formed into squares, but were unable to hold their formation against an attack by the Baden dragoons. The squares broke, and the infantry surrendered. Most of the 500 infantry in the squares were taken prisoner, and in total the Austrians suffered around 800 casualties during the fighting.
Only after this action was over did Schustekh finally receive Hiller's order to send his covering forces west. By this time Schustekh barely had any covering forces left, and so he sent one infantry battalion, III/ Klebek, and two hussar squadrons towards Riedau. This force attempted to push the French out of Riedau, but failed at heavy cost. Schustekh moved the remained two battalions of the Klebek infantry regiment into the fight, but they had no more success. Eventually the French were ordered to rejoin the bulk of Massena's corps, and marched off to Raab.
Further north Carra Saint-Cyr's division won a final French victory. Late in the day they were ordered to advance towards Waizenkirchen. After a rapid night march the French captured the town and its bridge almost without a fight.
A similar pattern of minor actions took place on 2 May, before on 3 May the two armies came together in a more important battle, at Ebersberg or Ebelsberg, on the Traun River.