Of the last the Division which had been left behind on the Spanish frontier formed the nucleus, to which was added a body of very superior troops, consisting of four regiments of marines; later the two cavalry divisions also joined. General Trochu, who had been made Governor of Paris, had taken back with him thither eighteen battalions of Gardes-Mobiles, they having already given such proofs of insubordination that it was thought unsafe to confront them with the enemy.
The Emperor had arrived in Châlons and had placed Marshal MacMahon in command of the newly-formed army. In the French Head-quarter it was not unnaturally assumed that Marshal Bazaine was in retreat from Metz. By an advance of the Army of Châlons merely to Verdun the armies could form a junction with each other in the course of a few days, and so a fighting force be formed which might make head against the hitherto victorious enemy. On the other hand, MacMahon had to concern himself with the duty of covering Paris, and that capital, no less than his own right flank, was threatened by the appearance of the Crown Prince of Prussia's army on the Meuse.
For the attainment of a decision between advancing and retiring, it was beyond everything necessary to know the direction which Marshal Bazaine might have taken.
On the 18th tidings had come from him, that he had maintained his position in a battle about Rezonville, but that his troops had to be supplied with ammunition and supplies before they could renew the march. From this it seemed only too probable that the communications of the Army of the Rhine were already threatened; and MacMahon determined to march on Rheims, whence he could either reach Paris, though by a somewhat circuitous route, or move in the direction of the other army.
But when it became known that the Crown Prince of Prussia's army had not even been near Metz, and that Prussian cavalry had already appeared before Vitry, the Marshal could not deceive himself as to the danger involved in the latter alternative. With sound judgment, therefore, he stood out against the order of the Empress and the Ministry to undertake that enterprise; he determined against it, and announced his resolution to march to Paris. Under its walls he could accept a battle with advantage, since the fortifications, even in the event of defeat, assured a safe retreat and precluded pursuit.
Further reports from Metz did not afford a clear insight into the situation there. Also on the 18th, "the army had held its position," the narrative ran—only the right wing had changed front. "The troops required two or three days' rest," but the Marshal "counted still on being able to move out in a northerly direction," and fight his way to Châlons by the Montmédy—Ste. Menehould route, if this road was not strongly held by the enemy. In that case, he would march on Sedan, and even by Mézières, in order to reach Châlons.
Bazaine might already have committed himself to the movement thus indicated, and therefore Marshal MacMahon, who was not the man to leave his fellow-soldier in the lurch, instead of marching on Paris, set forth on the 23rd in the direction of Stenay.
The suddenness of this decision caused all the preparations for the undertaking to be left unexecuted. At the end of the first day's march the troops reached the Suippe late in the evening in pouring rain. They lacked every necessary, and two Corps remained entirely without food. The Marshal was therefore forced to move his army further northward to Rethel, where large magazines of provisions had been established, and where the railway facilitated the bringing up of stores. Even on the third day's march the army had made little progress eastward. The left wing remained at Rethel, the right reached the Aisne, near Vouziers. On August 26th the main army was still standing between Attigny and Le Chêne on the Ardennes canal, while the VIIth Corps and a regiment of Hussars lay in front of Vouziers for the protection of the right flank.
While the French army was thus marching eastward by a wide détour, the German forces, which had been put in motion at the same time, were for their part marching due westward.
According to orders issued from the supreme Head-quarter at Pont à Mousson, the advance on the enemy, supposed to be at Châlons, was to be effected in such manner that the IIIrd Army, marching on the left of the Army of the Meuse, should have the start by a day's march, so that the enemy, wherever he might stand halted, could be struck simultaneously in front and on his right flank, and thus forced away northward from the direction of Paris. The two armies were to converge as they advanced, and to reach the line of Ste. Menehould—Vitry on the 26th.