The Battle of Montmirail was fought on the 11th of February 1814, the very next day after Napoleon's victory at Champaubert. It was fought by Napoleon to capitilise on the victory he had won over General Olsuffiev's Russian corps the previous day and by moving quickly, to retain the initiatve he had won by placing himself astride the Army of Silesia's communications.
Prelude
After his destruction of General Olsuffiev's Russian corps at the Battle of Champaubert , Napoleon now occupied the perfect 'Central Position' and was ideally placed to move either against General's Yorck and Sacken to the west who were chasing Marshal Macdonald, or to move against Marshal Blucher, the 'Old Forward' himself, who lie to his other side to the east
Napoleon chose to move against Sacken and Yorck in the west, since he surmised that if he dashed at Blucher, the old Prussian would merely withdraw before him upon his supply lines and his blow would fall upon empty air, whilst Sacken and Yorck meanwhile would simply make good their escape over the River Marne. In that event, his initiative he had won would be lost.
" I shall march to defeat Sacken at Montmirail tommorow," Napoleon confided. Therefore, having made his decision to eliminate the enemy forces lying west, Napoleon ordered Marshal Marmont with 4, 000 men to stay put at Champaubert to watch Blucher. To Macdonald, retreating towards Meaux, he instructed to hold fast and to resume the offensive, moving to capture Chateau Thierry and deny the enemy an escape route over the Marne. Sending orders to both Oudinot and Mortier to be ready to march to the sound of the guns, Napoleon then led the rest of his army through the night and the deep mud to Montmirail.