He decided to plod on towards the Meuse.
The Germans were now on the alert to entrap this army that exposed its flank in a long line of march near to the Belgian frontier.

Their ubiquitous horsemen captured French despatches which showed them the intended moves in MacMahon's desperate game; Moltke hurried up every available division; and the elder of the two Alvenslebens had the honour of surprising de Failly's corps amidst the woods of the Ardennes near Beaumont, as they were in the midst of a meal.

The French rallied and offered a brisk defence, but finally fell back in confusion northwards on Mouzon, with the loss of 2000 prisoners and 42 guns (August 30).
This mishap, the lack of provisions, and the fatigue and demoralisation of his troops, caused MacMahon on the 31st to fall back on Sedan, a little town in the valley of the Meuse.

It is surrounded by ramparts planned by the great Vauban, but, being commanded by wooded heights, it no longer has the importance that it possessed before the age of long-range guns of precision.

The chief strength of the position for defence lay in the deep loop of the river below the town, the dense Garenne Wood to the north-east, and the hollow formed by the Givonne brook on the east, with the important village of Bazeilles.