With the Prussian army routed at Jena-Auerstedt, Napoléon occupied the major cities of Germany and marched east in pursuit of the remaining forces opposed to him. These were largely Russians under the command of the frail 68-year-old Field Marshal Count Mikhail Kamensky. The old marshal was unwilling to risk battle and continued to retreat, leaving the Grande Armée free to enter Poland almost unopposed. Nevertheless, as the French pressed aggressively eastward across the Vistula, they found the Russians defending the line of the Wkra River. The French seized a crossing over the Wkra on 23 December at the Battle of Czarnowo. Russian resistance soon stiffened and on 26 December the two armies clashed at the Battles of Pułtusk and Gołymin. After these fierce engagements Napoléon's troops took up winter quarters in Poland to recuperate after a victorious but exhausting campaign.