The combat of Mohrungen (25 January 1807) saw Bernadotte's corps defeat part of a Russian army that was attempting to attack the isolated left wing of Napoleon's army in Poland in the winter of 1806-7.
After the inconclusive battles of Pultusk and Golymin (26 December 1806) Napoleon decided to go into winter quarters east of the Vistula. Most of his corps were concentrated in the area to the north and north-east of Warsaw, but his line ran all the way to the Baltic, with Bernadotte's corps on the extreme left and Ney's corps next in line. Both men were expected to remain in their winter quarters, but Ney ignored his orders and sent some of his men north towards Konigsberg. Napoleon blamed Ney for provoking the Russians into action, although this wasn't actually the case.
At the end of December the main Russian army, under Buxhowden and Bennigsen, was camped to the north-east of the main French concentration, on the Narew River. Much to Napoleon's surprise the Russians decided to launch a fresh offensive. They decided to move north then turn west to attack Bernadotte's isolated corps, using the Forest of Johannesburg to hide their movement. The French were almost entirely in ignorance of this move until 19 January, when the Russian advance guard ran into some of Ney's cavalry at Schippenbeil, on the River Alle. Over the next two days Ney withdrew south-west to Neidenburg, and sent messages to Bernadotte to inform him of the Russian advance.
Bernadotte responded quickly and ordered his scattered corps to concentrate. Rivaud's division was to concentrate at Osterode, Drouet's further north at Saalfeld, from where it was to advance to Mohrungen (modern Morag) and Dupont's division at Preussisch Holland (Paslek), northwest of Mohrungen.
By noon on 25 January Bernadotte had reached Mohrungen. He had nine infantry battalions and eleven cavalry squadrons with him, most from Dupont's division and Drouet's division. This put him in the path of the Russian right, which was advancing towards Liebstadt.
The Russian force, under General Markov, advanced to the village of Georgenthal (now Jurki, two miles to the north of Mohrungen). He placed two infantry regiments in his first line, and another regiment in his second line. Two more infantry battalions were sent forward to the hamlet of Pfarrersfeldchen (Pastor's field, modern Plebania Wólka, just to the north of Mohrungen). A regiment of hussars was placed in front of this advance guard. Markov also had five battalions of jägers and a large number of Cossacks.
Bernadotte decided to attack the Russians. Dupont was ordered to rush to the battlefield and attack the Russian right wing, while Bernadotte himself launched a frontal assault on the Russian position.
The fighting began at around 1pm when the French cavalry attacks the Russian hussars. The Russians had the best of this early fighting, but were eventually driven off by the French artillery. The French cavalry then advanced towards the Russian lines, but they too were stopped by hostile artillery.
Bernadotte then decided to launch an infantry assault on the Russians at Pfarrersfeldchen. The first French attack used a battalion from the 9th Line and the 1st battalion of the 27th Line. The 9th was held off by the Russians, but the 27th advanced into some woods to the right of the hamlet. During the fighting in the woods this battalion temporarily lost its eagle, but was able to recover it.
The attack was reinforced by the 2nd battalion of the 27th Line and the 8th light, with the 94th line in support. This was enough to force the Russians out of Pfarrersfeldchen. Bernadotte then attacked the main Russian position at Georgenthal, while Dupont's troops had now arrived and were attacking the Russian right. Under pressure from both sides Markov decided to retreat. Bernadotte began to pursue the retreating Russians, but was forced to abandon the chase when he heard gunfire from Mohrungen, now in his rear. A force of Russian cavalry had arrived at the undefended village from the east and captured the French baggage. This cavalry force took 360 French prisoners and freed 200 Russians and Prussians, but was itself forced to retreat when Bernadotte appeared on the scene.
The French admitted to having suffered 700-800 casualties at Mohrungen, while claiming to have inflicted 1,600 casualties on the Russians. Other sources give higher casualty figures, perhaps as many as 2,000 on each side.
After the fighting was over Bernadotte retreated, allowing Bennigsen to occupy Mohrungen. He decided to rest for a few days to recover from the ten day march around the French left flank, and was now convinced that he had defeated Bernadotte's corps and was about to achieve his main aims. Instead on 1 February he received a captured copy of Napoleon's orders for a counter-attack that would have seen the French attack the Russian's left and rear and probably led to a disastrous defeat. This advance warning gave Bennigsen the time he needed to cancel his advance and order a retreat. The French caught them at Jankovo (3 February 1807) and more famously at Eylau (8 February 1807). Both of these battles were inconclusive, and after Eylau both sides went into winter quarters, where they remained until the summer.