Based on Soviet records, over 10,000 German soldiers continued to resist in isolated groups within the city for the next month. Some have presumed that they were motivated by a belief that fighting on was better than a slow death in Soviet captivity. Brown University historian Omer Bartov claims they were motivated by National Socialism. He studied 11,237 letters sent by soldiers inside of Stalingrad between 20 December 1942 and 16 January 1943 to their families in Germany. Almost every letter expressed belief in Germany's ultimate victory and their willingness to fight and die at Stalingrad to achieve that victory. Bartov reported that a great many of the soldiers were well aware that they would not be able to escape from Stalingrad but in their letters to their families boasted that they were proud to "sacrifice themselves for the Führer".