Excerpts from Books and Wikipedia
"With the 1807 Tilsit agreements breaking down, Napoleon decided that defeat of the Russian Empire would ensure that it worked with him and his economic warfare against his most enduring enemy, Britain.…It was Napoleon's largest army, comprising some 690,000 men….The Russian armies, commanded by Barclay de Tolly and Mikhail Kutusov, initially retreated before the French onslaught. They understood that keeping the Russian army intact was vital, and that distance, lack of supplies, and weather would defeat the French." [1001 Days] "Napoleon was defeated not so much by the Russian winter as by his failure to keep the Grand Army properly supplied. The retreat from Moscow destroyed the Army. Of the 610,000 men who had at one time or another crossed the Russian frontier, 100,000 or so recrossed it." [Hobsbawm; Revolution, p. 87]