Napoleon slowly cam to accept the fact that the Tsar would not come to terms, knowing as he did the weaknesses of the French position and the impossibility of a further effective French offensive that year. To advance was to die; to stay put was to rot; the only course open was to retreat, in the hope of saving the greater part of the army. After the failure of his second mission to the Tsar (it returned to Moscow on October 17), the Emperor at last made up his mind. The Grande Armee would retire and make for the well-provisioned depots of Smolensk by the southern route, crushing Kutusov at Kaluga on its way if this proved necessary. On the 18th, the corps commanders were ordered to be ready to leave Moscow on the 20th. The days of lingering were over. Napoleon accepted that Alexander had called his bluff.
Lauriston leaving Russian camp
~ D. Chandler, The Campaigns of Napoleon p. 818-819