It pitched troops of the Kingdom of Westphalia against the Black Brunswickers under Frederick William, Duke of Brunswick-Wolfenbüttel, but ended in a tactical draw.

Resisting this, Frederick William, equipped 2,000 troops at his own expense and offered them and his services to Emperor Franz I of Austria in the Convention of Vienna on 25 February 1809.

The troops were joyfully welcomed by the population, but on the morning following their arrival Frederick William received word that a 5,000-strong Westphalian division commanded by General Reubell was approaching from the north of the city.

At 14:00 the "Black Horde" moved toward Ölper, reinforced by about 200 citizens of Braunschweig, who were equipped with material looted from Halberstadt.

At the end of the day the Westphalians had taken the battlefield, but they withdrew from Ölper after nightfall and did not defeat the "Black horde", nor stop their further movement to the North Sea.

On 2 August the Brunswick Corps broke through and marched on through Celle, Hanover, Nienburg and Delmas, while pursued by Reubell's troops.

Duke Friedrich Wilhelm's troops entered British service, and they fought from 1810 to 1814 under the command of Wellington in the Peninsular War.