The combat of Biezun (23 December 1806) saw the defeat of a Prussian attempt to recapture Biezun on the Ukra River, a key position that connected the Prussians to their Russian allies.
After the disasters at Jena and Auerstadt (14 October 1806) the French pursued and destroyed most of the remaining Prussian armies. By the end of November the only remaining Prussian field force was a corps of about 6,000 men under General Lestocq, which was based in eastern Prussia. Lestocq had attempted to hold Thorn on the Vistula, but his Russian allies decided not to hold that river and so Lestocq was forced to retreat east towards the upper reaches of the Ukra River. By 18 December his main force was at Lautenburg, with his left flank running south towards Biezun on the Ukra. This was an important position as it connected the Prussian and Russian lines.
Lestocq was badly outnumbered by his direct French opponents. The French left was made up of Ney's corps two thirds of Bernadotte's corps and Bessières' 2nd cavalry reserve. On 19 December Grouchy's 2nd Dragoon division captured Biezun, threatening to split the Prussian and Russian lines. Lestocq decided to send two infantry regiments and three cavalry regiments to try and retake this key position.
By 23 December Bessières had joined Grouchy. The bridge over the Ukra was defended by two companies of infantry, supported by a large part of Bessières's 6,000 cavalry.
The Prussians arrived at Biezun at eight in the morning of 23 December and occupied the small village of Karniszyn, on the east bank of the Ukra. Bessières ordered Grouchy to attack the Prussian positions with his division. The Prussians were caught before their line was fully formed, and they were swept away by Grouchy's charge. The Prussians were pursued for ten miles and lost 500 prisoners, 5 guns and two standards during the battle. The Prussians were forced to retreat north-east towards Soldau, with Marchand's division in pursuit. Bernadotte joined the pursuit and within a few days the Prussians were retreating towards Konigsberg and away from the Russians.