After conquering Prussia in the autumn of 1806, Napoleon entered Poland to confront the Russian army, which had been preparing to support the Prussians until their sudden defeat. Crossing the river Vistula, the French advance corps took Warsaw on 28 November 1806. Kamensky had reversed the Russian retreat, and ordered an advance to support the troops on the River Ukra. Because of this, the French experienced difficulty crossing the river and it was not until Davout forced a crossing near the junction of the Wkra and the Narew on 22 December that the French were able to advance.