To Generals in command of Corps d'Armée, such as Thielemann and Pirch, a little reflection upon Grouchy's critical position must have led to a similar conclusion. The inactivity of the former, during the afternoon and evening of the 19th, is probably to be explained by his having satisfied himself that the longer Grouchy continued in the vicinity of Wavre, the greater became the chance of his retreat being cut off by a portion of the Allied Armies; which, in their advance, would reach the Sambre much sooner than it would be in the power of the French Marshal to do: and that, therefore, it would be injudicious on his part to attempt to force the latter from the position, which appearances induced him to believe he still occupied with his entire force, on the Dyle. He may also have been strengthened in this opinion by the circumstance of his not having received any positive instructions as to his future dispositions, or any reinforcements to secure for him a preponderance over Grouchy.
With Pirch, however, the case was very different. He received distinct Orders, on the evening of the 18th, to march at once from the Field of Waterloo, and continue his movement during that night, so as to cut off Grouchy's retreat upon the Sambre. It has already been explained, that on reaching Mellery, at eleven o'clock on the following morning, he halted to give his troops rest; that he subsequently ascertained, through Lieutenant Colonel Sohr, who had been despatched, during the march, with his Cavalry Brigade to reconnoitre on the left, that the French occupied the Defile of Mont St Guibert in force. This intelligence might have satisfied him that Grouchy had not yet reached Namur; but, if he entertained any doubts on that point, these could easily have been settled by means of a reconnoitring party, detached from Mellery, by Gentinne, and St Gery, to Gembloux, a distance of seven miles. He would then have learned, that no portion whatever of Grouchy's force had hitherto crossed this line, in retreat; that he had, consequently, gained considerably on his rear, and had it in his power, after allowing a few hours rest to his troops, to march them by the high road which leads directly from Mellery into the high road near Sombref, and to anticipate Grouchy in the possession of Namur.
In this case, Grouchy, on approaching the latter place, and finding it occupied by Pirch, would, in all probability, have hesitated to risk the loss of so much time as an attempt to force the town and the Pont de Sambre would necessarily incur, and have preferred endeavouring to pass his troops across the Sambre by some of the bridges and fords between Charleroi and Namur, and retire upon either Philippeville or Dinant; but with a Prussian Corps d'Armée at each of these points, and another in his rear, this would have been, to say the least of it, a most hazardous undertaking; and if he attempted to cross the Meuse below Namur, his chance of regaining Napoleon's Army would have been still more remote.
But setting aside the circumstance of Pirch's not having, in this manner, taken due advantage of the position in which he stood relatively with Grouchy during the 19th; and passing to the fact, that he first learned, at five o'clock on the morning of the 20th, whilst still at Mellery, that the Enemy was retiring along the high road from Gembloux to Namur, pursued by Thielemann's Cavalry: it seems strange that, inferring, as he must naturally have done, that Grouchy would only endeavour to hold out long enough at Namur to effect his passage by the Pont de Sambre, and to cover his retreat to Dinant, he did not immediately move off by his right, and push his troops across the Sambre by some of the bridges and fords higher up the stream; and then, marching in the direction of Profondeville, under cover of the Wood of Villers within the angle formed by the confluence of the Sambre and the Meuse, intercept Grouchy's retreat through the long and narrow Defile in which the road to Dinant winds by the side of the last mentioned river. The situation in which Grouchy would have been placed by a movement of this kind—his troops in a long, narrow, precipitous Defile, obstructed in front by Pirch, and attacked in rear by Thielemann—would have been perilous in the extreme.
Pirch probably felt that his Corps d'Armée, part of which was then attached to the Army pressing the Enemy by the Charleroi road, was not equal to cope with Grouchy's troops; but in the case here supposed, by judiciously disposing his force then present so as to command the Defile at some favourable point in its course, he would have secured for himself an advantage which, under such circumstances, would have fully compensated for his deficiency in regard to numbers.
The scattered remnants of the main French Army continued to be hurried forward in wild confusion across the frontier. Some of the fugitives hastened towards Avesnes, others to Philippeville: whilst a very great proportion of them sought no temporary rest of this kind, but, throwing away their arms, fled into the interior, to return to their homes; the Cavalry, in many instances, disposing of their horses to the country people. Several of the superior Officers hastily collected such of the troops as appeared better disposed, and conducted them in the direction of Laon. Napoleon reached the latter town in the afternoon of the 20th. After conferring with the Préfet, he desired M. de Bussy, one of his Aides de Camp, to superintend the defence of this important place; and despatched General Dejean to Avesnes, and General Flahaut to Guise.
In the mean time, a body of troops had been discerned in the distance, moving towards the town. Napoleon sent an Aide de Camp to reconnoitre it; when it proved to be a Column of about three thousand men, which Soult, Jerome, Morand, Colbert, Petit, and Pelet had succeeded in rallying and preserving in order. Napoleon now appeared intent upon remaining at Laon until the remainder of the Army had reassembled: but he subsequently yielded to the force of the arguments expressed in opposition to this determination by the Duke of Bassano and others who were present, and took his departure for Paris; purposing, at the same time, to return to Laon on the 25th or 26th of the month.
The following was the general disposition of the respective Armies on the evening of the 20th:—