Napoleon's Defeat in Russia

History has taught us that Napoleon, in his invasion of Russia in 1812, marched into Moscow with his army largely intact and retreated only because the citizens of Moscow burned three-fourths of the city, depriving the army of food and supplies. The harsh Russian winter then devastated the army as it retreated. The Russians’ victory, commemorated by Tchaikovsky’s 1812 Overture, was one of the great upsets of military history...

...What incredible circumstances could have caused the defeat of one of the greatest armies on the European continent, led by one of the greatest generals of all time?