effects of extreme cold, gives the following account:

Soldiers unable to go further fell and resigned themselves to death, in
that frightful state of despair which is caused by the total loss of moral
and physical force, which was aggravated to the utmost by the sight of
their comrades stretched lifeless on the snow. During a retreat so
precipitate and fatal, in a country deprived of its resources, amid
disorder and confusion, the sad physician was forced to remain an
astonished spectator of evils he could not arrest, to which he could apply
no remedy. The state of matters remarkably affected the moral powers. The
consternation was general. Fear of not escaping the danger was very
naturally allied with the desperate idea of seeing one's country no more.
None could flatter himself that his courage and strength would suffice so
that he would be able to withstand privations and sufferings beyond human
endurance. Italians, Portuguese, Spaniards, those from the temperate and
southern parts of France, obliged to brave an austere climate unknown to
them, directed their thoughts toward their country and with good reasons
regretted the beauty of the heaven, the softness of the air of the
regions of their birth.

Nostalgia was common.... The army was but three days from Smolensk when the
heavens became dark, and snow began to fall in great flakes, in such a
quantity that the air was obscured. The cold was then felt with extreme
severity; the northern wind blew impetuously into the faces of the soldiers
and incommoded many who were no longer able to see. They strayed, fell into
the snow--above all, when night surprised them--and thus miserably
perished.

Disbanded regiments were reduced to almost nothing by the loss of men
continually left behind either on the roads or in the bivouacs.