At the battle of Mars-la-tour, two prussian corps encountered the entire French army of the Rhine. Although at the day's end, the French were victorious, Marshal Bazaine ordered a retreat. Two days later, the prussian victory of Gravelotte trapped the 180.000 men of Bazaine in Metz.
What if, instead to retreat to Metz, they continued, as ordered by the Emperor, the retreat towards Châlons?
I doubt that the Germans could inflict to a French army of 260.000 men a fatal blow, as at the OTL battle of Sedan.