Excerpts from Books and Wikipedia
The 1807 Tilsit agreements were breaking down--Russia was not doing enough to blockade Britain economically. Napoleon decided to invade. The Russian army retreated to draw the French into a trap--no supplies, long distance from France, and an extreme winter. [Furtado: 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]