In Panzer Battles, von Mellenthin has plenty to say about the psychology of the Russian soldier:
No one belonging to the cultural circle of the West is ever likely to fathom the character and soul of these Asiatics, born and bred on the other side of the European frontiers.
(I am of course aware that the Slavs migrated into Russia from the west, and were originally a European people. But the Mongol invasion of 1241, and the two centuries of domination which followed, gave an Asiatic twist in the Russian outlook and character, a development accentuated by the policy of the Tsars.)
Yet the Russian character must contain the key to an understanding of their soldierly qualities, their achievements, and their way of fighting. The human heart, and the psychology of the individual fighting man, have always been the ruling factors in warfare, transcending the importance of numbers and equipment. This old maxim held good during World War II, and I think it will always to so.
Of course, he also cites Russian numbers — of men and tanks — as the key to their victory, and later he cites the Americans’ overwhelming advantages in air power and artillery as the key to their victory, not superior fighting spirit.
Anyway, he continues his description of the Russian soldier as very, very foreign:
There is no way of telling what the Russian will do next; he will tumble from one extreme to the other. With experience it is quite easy to foretell what a soldier from any other country will do, but never with a Russian. His qualities are as unusual and many-sided as those of his vast and rambling country. He is patient and enduring beyond imagination, incredibly brave and courageous — yet at times he can be a contemptible coward. There were occasions when Russian units, which had driven back German attacks with ferocious courage, suddenly fled in panic before a small assault group. Battalions lost their nerve when the first shot was fired, and yet the same battalions fought with fanatical stubbornness on the following day. The Russian is quite unpredictable; today he does not care whether his flanks or threatened or not, tomorrow he trembles at the idea of having his flanks exposed. He disregards accepted tactical principles but sticks to the letter of his field manuals. Perhaps the key to this attitude lies in the fact that the Russian is not a conscious soldier, thinking on independent lines, but is the victim of moods which a Westerner cannot analyze. He is essentially a primitive being, innately courageous, and dominated by certain emotions and instincts. His individuality is easily swallowed up in the mass, while his powers of endurance are derived from long centuries of suffering and privation. Thanks to the innate strength of these qualities, the Russian is superior in many way to the more conscious soldier of the West, who can only make good his deficiencies by superior mental and moral training.
He reiterates their contempt for life or death, their fondness for “Little Mother Russia,” but not for the Communist regime, their indifference to seasons, and their independence from food supplies — which are offset by dullness, mental rigidity, and indolence.