Whilst Bülow's Reserve Cavalry, following the Thirteenth Infantry Brigade, was passing through Wavre, French Cavalry had penetrated between the Rear Guard of this Corps, at Vieux Sart, and the Detachment under Lieutenant Colonel Ledebur at Mont St Guibert. The 2nd Pomeranian, and the 1st Silesian, Landwehr Cavalry were immediately detached from the Reserve Cavalry of the Corps, to aid in checking the advance of the Enemy.
The Prussian Lieutenant Colonel Ledebur, who was still at Mont St Guibert, having received intelligence of the approach of the French, decided on commencing his retreat towards Wavre. Lieutenant Colonel Sohr, who had fallen back early in the morning from Mont St Guibert, sent 150 Cavalry and two guns of Horse Artillery as a reinforcement to Ledebur. The latter now succeeded in forming a junction with the two Cavalry Regiments detached from the Reserve, as also, subsequently, with the Cavalry Brigade under Sohr, after a slight affair with the Third French Corps (Vandamme's), whilst making good his retreat to Auzel.
Pirch's Corps (the Second) broke up from its position between St Anne and Aisemont, on the right bank of the Dyle, about noon, for the purpose of passing the Defile of Wavre. The 1st Battalion of the 14th Regiment, which occupied this town, was relieved by a Battalion of the 30th Regiment, belonging to the Third Corps (Thielemann's). Pirch had just put his Corps in motion, with a view to cross the Dyle by the town of Wavre, when the approach of the Enemy was announced. The Defile was crowded with the troops; the progress of their march could not be otherwise than slow; and at this moment Lieutenant Colonel Sohr, whose Brigade formed the Rear Guard of the Corps, sent in word, that the Enemy presented a force of six Regiments of Cavalry, ten pieces of Artillery, and two strong Columns of Infantry.
The Wood of Sarats, close to the Farm of Auzel, was now occupied by some Battalions of the Eighth Brigade, the command of which had devolved upon Colonel Reckow. Pirch placed the whole of the Rear Guard under the Orders of General Brause, the Commander of the Seventh Brigade, and reinforced Lieutenant Colonel Sohr with the 11th Hussars and four pieces of Horse Artillery. Brause posted the remaining Battalions of the Eighth Brigade in rear of the Wood, and the three Regiments of Cavalry on the right, with the Foot Battery No. 12 in their front. The Seventh Brigade, deployed into line, remained in reserve.
Lieutenant Colonel Ledebur retired slowly before the Enemy, and formed a junction with the Eighth Brigade, under Colonel Reckow; who maintained his position until three o'clock in the afternoon, against the Advanced Guard of Vandamme's Corps. Between three and four o'clock, General Brause ordered the retreat. Lieutenant Colonel Sohr crossed the Bridge at the Mill of Bierge, which was occupied by two Companies of the 2nd Battalion of the 14th Regiment; and then followed the Reserve Cavalry of Pirch's Corps, to which his Brigade belonged, but which he did not overtake until he reached the Field of Waterloo. The Enemy did not advance with much vigour; the retreat was conducted with perfect order, and the Fusilier Battalion of the 1st Pomeranian Landwehr, under Major Krüger, distinguished itself on the occasion. After the passage of the river had been effected, the 1st Battalion of the Elbe Landwehr remained at Bierge until the Bridge was destroyed and the Mill set on fire. The 11th Hussars and the 2nd Battalion of the Elbe Landwehr were posted in observation of the passages across the Dyle, and did not rejoin the Corps before the following day.
Blücher had quitted Wavre before eleven o'clock in the forenoon, and repaired to the vicinity of Limale, in order to make himself acquainted with the nature of the country in the direction of St Lambert. Whilst here, he received intelligence of the approach of the Enemy towards Wavre. Colonel Clausewitz, Chief of the Staff of the Third Corps, was immediately made the bearer of an Order for Thielemann to defend the position at this place, in the event of the Enemy advancing in force; but, should the latter cross the Dyle higher up the stream, or not appear in great strength (a point concerning which nothing positive was then known), he was to leave only a few Battalions in position at Wavre, and to follow the main Army, with his Corps, as a Reserve, in the direction of Couture.
Grouchy's movements, on the 17th and 18th, form so striking a feature in the history of this Campaign, and exercised so important an influence upon the fate of the decisive Battle of Waterloo, that it becomes an essential point in the study of that history, to examine how far he complied with, and carried into effect, the instructions received from his Master, and to what degree his proceedings, consequent upon his ascertaining the direction of the Prussian retreat, coincided with the general plan and object of Napoleon's operations. On a reference to the account of his transactions during the 17th, given in the despatch written at ten o'clock on that night, it appears he was completely ignorant of the line by which the principal mass of the Prussian Army had retreated, namely, that of Tilly and Gentinnes, by Zieten's and Pirch's Corps, although his Cavalry had driven back the Prussian Detachment from the latter place to Mont St Guibert, but from whence it appears to have been withdrawn in the night. His attention seems to have been much less devoted to this quarter than it was to his Right, in which direction he detached as far as Perwès. The main body of his forces did not proceed further than Gembloux on the 17th, that is, about five miles from the Field of Ligny.
Upon a first consideration, we are strongly impressed by the striking contrast between this march, in pursuit of a defeated Army, which had commenced its retreat on the previous night, and which presented no check to the advance, and the march of Napoleon from the same Field, by Quatre Bras and Genappe, as far as La Belle Alliance, in front of the Waterloo position, a distance of about sixteen or seventeen miles; and this, too, in rear of a victorious Army, with a Cavalry Rear Guard boldly and successfully impeding the advance of its pursuers. It must, however, be taken into account, that in one most important respect Napoleon possessed a decided advantage over Grouchy—an advantage, the magnitude of which increased with every moment that elapsed after the wet weather had set in; for while the former moved the whole distance along a paved high road, the latter had to proceed entirely by cross roads, which may more properly be designated common field roads. It is to this particular point that both Grouchy and Gérard refer in justification of the late arrival of the Infantry at Gembloux. Nevertheless, Grouchy detached considerably to his Right, with his Cavalry, misled by the same idea which had prevailed with Napoleon, that Blücher had retreated upon the Meuse; and the very circumstance of his Dragoons having reached Perwès on the night of the 17th, proves that had he organised a more extended, more combined, and more energetic reconnaissance, with the sixty five Squadrons of Cavalry which had been placed under his orders, he might have connected his operations on the right of the Dyle with those of Napoleon on the left of that river, by occupying the line of Nil St Vincent, Corbaix, Mont St Guibert, and the Bridge of Moustier. The only check he would have experienced would have been at the Prussian Post of Mont St Guibert; which, however, in the case of an active reconnaissance, as above, might have been attacked by a strong Detachment in front, and turned by Corbaix on its left.
To show the connection which this disposition would have secured with Napoleon's movements, it is only necessary to state, that the Third Cavalry Division, under Lieutenant General Domon, had been detached from the Emperor's Column to reconnoitre the country between the Dyle and the high road to Brussels; and that the 4th Regiment of Chasseurs à Cheval pushed as far as the Bridge of Moustier, on which line its Skirmishers exchanged a few carabine shots with some Prussian Dragoons, who did not, however, appear willing to engage further with them. It was by means of this reconnaissance that Napoleon ascertained the retreat, through Tilly and Gentinnes, of the principal Prussian Column, consisting of Zieten's and Pirch's Corps, although the line by which they retired was undiscovered by Grouchy, in whose immediate sphere of operations it was situated.