The Israeli historian Omer Bartov wrote that on the Eastern Front, it was belief in National Socialism that allowed the Wehrmacht to continue to fight, despite enormous losses.[63] Bartov argued that the claim that it was "primary group loyalty", by which men are motivated to fight by loyalty towards their comrades in their unit with little thought to the cause that one is fighting for, cannot possibly have been what motivated the Wehrmacht to fight on the Eastern Front.[64] Bartov wrote that on the Eastern Front, the Wehrmacht was taking such heavy losses that there were no "primary groups" for men to give their loyalty to, and that only a belief in National Socialism could explain why the Wehrmacht continued to be so aggressive and determined on the offensive, and so dogged and tenacious on the defense, despite often very high numbers of dead and wounded.[64] The Bartov thesis was endorsed by the American historians Alan Millet and Williamson Murray, who wrote that, by early 1944, "group cohesion alone" could not explain why the German soldiers carried on fighting: