The invasion commenced on 24 June 1812. Napoleon had sent a final offer of peace to Saint Petersburg shortly before commencing operations. He never received a reply, so he gave the order to proceed into Russian Poland. He initially met little resistance and moved quickly into the enemy's territory. The French coalition of forces amounted to 449,000 men and 1,146 cannons being opposed by the Russian armies combining to muster 153,000 Russians, 938 cannons, and 15,000 Cossacks. The center of mass of French forces focused on Kaunas and the crossings were made by the French Guard, I, II and III corps amounting to some 120,000 at this point of crossing alone. The actual crossings were made in the area of Alexioten where three pontoon bridges were constructed. The sites had been selected by Napoleon in person. Napoleon had a tent raised and he watched and reviewed troops as they crossed the Niemen. Roads in this area of Lithuania hardly qualified as such, actually being small dirt tracks through areas of dense forest. Supply lines simply could not keep up with the forced marches of the corps and rear formations always suffered the worst privations.

http://en.wikipedia.org/wiki/French_invasion_of_Russia_%281812%29

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