At five o'clock on the morning of the 18th, Pajol started from Mazy, with Soult's Cavalry Division and Teste's Infantry Division, marching by St Denis and Grand Lez, to Tourinnes; where he was to await further Orders. At about eight o'clock, Excelmans' Corps of Heavy Cavalry, consisting of eight Regiments of Dragoons, was put in motion; and at nine o'clock, Vandamme's and Gérard's Infantry Corps began their march along one and the same road, by Sart à Wallain, upon Wavre. The Left of this Column was protected, towards the Dyle, by the advance of Maurin's Division of Light Cavalry, under General Vallin.
It was about half past ten o'clock, when Excelmans' Advanced Guard came up with the Prussian Rear Guard, on the road to Wavre. He immediately formed his troops in position, resting their Left upon the wooded ravine near the Farm of La Plaquerie, and their Right in the direction of Neuf Sart. While his Skirmishers were engaged with those of the Enemy, he sent the Chef d'Escadron d'Estourmel, to inform Marshal Grouchy of what was going on in front, and also to make known to him that the Prussian Army had continued its retreat upon Wavre during a part of the night and that morning, for the purpose of forming a closer communication with the Duke of Wellington's forces.
The march of the Third and Fourth Corps was greatly retarded by the bad state of the roads, and frequent halts were occasioned by the narrowness and miry nature of the Defiles. Gérard, having preceded the Column, reached Sart à Wallain at eleven o'clock, where he found Grouchy breakfasting in the house of M. Hollaërt, a Notary. In about half an hour after his arrival, Colonel Simon Lorière, who was acting as his Chief of the Staff, suddenly heard, while walking in the garden of the house, a distant but violent cannonade, of which he immediately went to apprise his General. Grouchy repaired at once to the garden, accompanied by Gérard, Vandamme, Excelmans, and several other Officers. He immediately called for M. Hollaërt, and asked him in what part of the country he considered this tremendous cannonade to be going on. The latter, pointing to the Forest of Soignies, replied that it must be in the direction of Planchenoit, Mont St Jean, and that vicinity.
Gérard then declared his opinion to be in favour of the expediency of marching in the direction of the cannonade, in order to connect the movements of the detached force more closely with Napoleon's operations; and offered to lead his own Corps towards the Battle. This measure was opposed by the Marshal, as also by General Baltus, of the Artillery, who represented the difficulties of a march in which this Arm might be compromised. On the other hand, General Valaze, Commanding Engineer of Gérard's Corps, after having coincided in the opinion expressed by the latter, observed that he had three Companies of Sappers, by aid of which he could remove many obstacles. Gérard then gave his assurance that he could at all events move on with the gun carriages and limbers.
Grouchy, however, stated his determination to act in conformity with his instructions; which were, to pursue and attack the Prussians, and never to lose sight of them. It had just been intimated to him that his troops had come up with a Rear Guard of the Enemy's Infantry, and he did not consider his information was sufficient to warrant the conclusion that Blücher was drawn up, in expectation of being attacked by him at Wavre; or that he would continue his retreat upon Brussels; or, that if, in manœuvring to effect his junction with Wellington, he would do so in front, or in rear, of the Forest of Soignies.
He has since declared, that he did not deem it his duty to follow the counsel given by Gérard, but to attack the Prussians; that to effect the proposed movement with the whole of his forces would have been acting contrary to his Orders; that to have detached only a portion of those forces in the direction of the Forest of Soignies, would have been to separate the two Corps of his Army by a river, whose waters were swollen by the rain, and whose banks were swampy, and thus have rendered their mutual support impossible, however essential it might have become; finally, that a war of inspiration appertains alone to the General in Chief, and that his Lieutenants must confine themselves to that of execution. Hence the march to Wavre was continued.
Whilst proceeding to the Advanced Guard, Grouchy received the despatch, dated from the Farm of Caillou, the 18th of June, at ten o'clock in the morning, acquainting him that the Emperor was going to attack the Anglo-Allied Army, in its position at Waterloo; desiring him to direct his movements upon Wavre in such a manner as to draw his forces nearer to those of Napoleon, and, especially, to keep up a close communication with the latter.
The receipt of these instructions was not followed by any immediate change in Grouchy's dispositions. He despatched no Cavalry force—not even a single Patrol—to watch any movements that might be attempted, or actually then in course of execution, by the Prussians, towards the Field on which the Emperor had intimated to him his intention of attacking the force under Wellington; and hence it is almost needless to add, that he neglected to establish that close and active communication with the main Army which was so essentially important for the accomplishment of the object of the general plan of operations, and to which Napoleon had especially directed his attention in the before mentioned despatch. His sole aim seemed to be a direct advance upon Wavre; and this he carried into execution without at all detaching to, or in any way manœuvring by, his left. On the contrary, upon arriving in person at the position occupied by Excelmans, he desired the latter to move to his right, and take post at Dion le Mont; and the ground thus vacated was shortly afterwards taken up by Vallin's Light Cavalry Division.