The engagement on the Regen or of Reinhausen of 17 April 1809 was a minor skirmish found on the north bank of the Danube opposite Regensburg that saw part of the Austrian advance guard clash with elements of Marshal Davout's isolated 3rd Corps.
At the start of the Franco-Austrian War of 1809 (War of the Fifth Coalition) Marshal Berthier was in command of the French and Allied armies in Germany, although Napoleon was directing operations from Paris. This system led to chaos, and by the time Napoleon arrived at the front the Allied armies were spread dangerously thin. Marshal Davout's 3rd Corps was in the most trouble, having been ordered east to Regensburg. This placed large parts of his corps on the north bank of the Danube, while the rest of the French army was some distance to the west, and on the opposite bank of the river.
By 17 April Austrian troops were closing in on Regensburg from north, south and east. The main Austrian army under Archduke Charles was the real threat, but was moving too slowly. A more direct threat came from elements of the two Austria corps that had begun the war in Bohemia. At the start of the war these corps had crossed into Bavaria and then turned south, and west, threatening to cut off Davout's line of retreat back to the north-west from Regensburg. After a pause on 13 April part of FML Klenau's advance guard of II Corps began to move south, and on 16 April it was approaching the French position at Regensburg.
In 1809 Regensburg was located on the south bank of the Danube, just opposite the point where the River Regen flows into it and connected to the north bank by a bridge to Stadt am Hof and Steinweg. These two towns were to the west of the Regen. Directly opposite them, on the east bank of the river, was the town of Reinhausen, with Weichs to its east (on the Danube) and Sallern to the north (on the Regen).
Davout had one infantry regiment in Regensburg itself and three voltigeur companies from the 17th Ligne in Reinhausen. His main forces in the area were focusing on protecting the road north, so Morand's division was in Stadt am Hof with St. Hilaire's division close by to the north-west.
The Austrian advance guard was never going to be strong enough to threaten the French position on the west bank of the Regen. Even so there was a real chance that they could have pushed the French out of their positions on the east bank. By the morning of 17 April Klenau was established in a position east of Sallern, but he didn't begin his attack until 1pm. Even then it was rather half-hearted. The strongest force, consisting of one battalion of infantry (III/ Zedtwitz), the 7th Jägers, the Riesch Dragoons and a battery of cavalry artillery, was used to push the French outposts out of Weichs. A second battery opened fire on Sallern. Reinhausen, at the heart of the position, was hardly touched. A few skirmishers were sent towards the town, and a few howitzer shells fired at it, but that was as close as the Austrians came. By the end of the day the Austrians suffered 57 casualties, while the French reported losing around 20.
As well as failing to push the French out of Reinhausen, Klenau also failed in his main role as a scouting force, not realising that most of Davout's corps was now concentrated around Regensburg. This lack of intelligence about their opponent's positions would cost the Austrians dearly over the next few days.
In the immediate aftermath of the battle Davout finally received orders directly from Napoleon, ordering him to cross to the south bank of the Danube and march west to join up with the rest of the army. At the same time the main Austrian army under Archduke Charles was marching north towards the Danube, leading to the first major battle of the campaign, at Hausen-Teugn (or Teugn-Hausen or Tengen) on 19 April 1809.