The combat of Kosen (20 October 1813) was a rearguard action during the French retreat from Leipzig.
On 18 October Napoleon realised that he was losing the battle of Leipzig, and prepared to retreat. Bertrand was ordered to lead his corps west to Weissenfels, to guard the potential crossing points over the Saale. Bertrand reached Saale without any problems, brushing aside what little opposition there was.
On 19 October the main French army abandoned Leipzig. The retreat began well but turned chaotic, especially after the last bridge leading west out of the city was blown early. Bertrand, at Weissenfels, was largely unaware of these disasters until the French columns began to appear at Weissenfels. This had a big impact on him, and when Napoleon reached Weissenfels he pleaded with him to leave the army so it would have more freedom of maneuver, without having to worry about protecting the Emperor (shades of 1812). Napoleon didn't agree with this, crossed the Saale, and spent the night of 20-21st resting in a pavilion in a vineyard on the west bank.
On 20 October the main body of the French army crossed the Saale at Weissenfels. The Allied pursuit was rather limited, but on 21 October Gyulai's corps reached the Saale at Kösen, south-west of Naumburg. The town of Kösen is on the eastern side of the Saale, with the suburb of Neu Kösen on the east bank. Just to the west of the river a very steep bank rises up to some heights that overlook the town. The west bank was defended by one of Bertrand's divisions.
Gyulai's corps found Bertrand's rearguard defending the heights on the left bank of the Saale at Kösen. The allies wanted to capture the bridge over the Saale, which was still intact. Gyulai's first attack swept across the bridge and almost reached the top of the heights.
Bertrand counterattacked and drove the Allies back across the bridge.
Gyulai then attacked in larger numbers, and took command of the force in person. This attack forced the French back across the Saale, and the Allies captured Neu Kösen.
The fighting dragged on to 10pm when Bertrand retreated. Both sides lost 1,000 killed and wounded, and the French also lost 649 prisoners.
On the same day Yorck attacked the French rearguard at Freiburg, where the French were crossing the Unstrut. The French fought off this attack and were able to complete the crossing intact.