Upon the departure of Napoleon, Grouchy ordered Vandamme and Gérard to get their Corps under arms, and to move them, in the first instance, to the junction of the Gembloux road with that to Namur; and having subsequently received intelligence that a considerable body of Prussians had passed through the former town, he desired that those two Corps should continue their movement upon that point. In the mean time, he repaired to the Advanced Posts of Excelmans' Dragoons, which were by this time beyond Gembloux. It was part of this Cavalry which followed Lieutenant Colonel Sohr, on the left. They merely threw out Skirmishers against him; and, as night set in, they abandoned the pursuit in this direction.

The Corps of Vandamme and Gérard did not reach Gembloux until very late in the evening. The former was posted in advance, the latter, in rear, of the town; near which also, and on the right bank of the Ormeau, was stationed the Sixth Light Cavalry Division, under General Vallin, who succeeded to the command, upon Lieutenant General Maurin being wounded at the Battle of Ligny. The First Brigade of Lieutenant General Chastel's Tenth Cavalry Division, consisting of the 4th and 12th Dragoons, under General Bonnemain, was pushed on to Sart à Wallain, and the 15th Dragoons (from General Vincent's Brigade of the Ninth Cavalry Division, under Lieutenant General Baron Soult), were detached to Perwès. From both these points, reports were sent into Gembloux that the Prussians had retired upon Wavre.

Pajol, with his Light Cavalry and Teste's Infantry Division, had returned from St Denis, between Namur and Gembloux, to the original position occupied by the latter in the morning, at Mazy, in the immediate vicinity of the Field of Ligny; a movement for which no satisfactory cause has ever been assigned.

The extent of information obtained by Grouchy concerning the Prussian retreat, and the nature of the dispositions which he adopted in consequence, will be best explained by the following despatch which he addressed to the Emperor:—

"Gembloux, le 17 Juin, "Gembloux, 17th June,
à dix heures du soir. at ten o'clock in the evening.
Sire,—J'ai l'honneur de vous rendre compte que j'occupe Gembloux et que ma Cavalerie est à Sauvenières. L'Ennemi, fort d'environ trente mille hommes, continue son mouvement de retraite; on lui a saisi ici un parc de 400 bêtes à cornes, des magasins et des bagages. "Sire,—I have the honour to report to you that I occupy Gembloux, and that my Cavalry is at Sauvenières. The Enemy, about thirty thousand men strong, continues his movement of retreat. We have seized here of his, a pen of 400 horned cattle, magazines and baggage.
"Il paraît d'après tous les rapports, qu'arrivés à Sauvenières, les Prussiens se sont divisés en deux Colonnes: l'une a dû prendre la route de Wavre, en passant par Sart à Wallain, l'autre Colonne paraît s'être dirigée sur Perwès. "It would appear according to all the reports, that, on arrival at Sauvenières, the Prussians divided themselves into two Columns: one of which took the road to Wavre, passing by Sart à Wallain; the other Column would appear to have been directed on Perwès.
"On peut peutêtre en inférer qu'une portion va joindre Wellington, et que le Centre, qui est l'Armée de Blücher, se retire sur Liège: une autre Colonne avec de l'Artillerie ayant fait son mouvement de retraite par Namur, le Général Excelmans a Ordre de pousser ce soir six Escadrons sur Sart à Wallain et trois Escadrons sur Perwès. D'après leur rapport, si la masse des Prussiens se retire sur Wavre, je la suivrai dans cette direction, afin qu'ils ne puissent pas gagner Bruxelles, et de les séparer de Wellington. "It may perhaps be inferred from this that one portion is going to join Wellington; and that the Centre, which is the Army of Blücher, is retiring on Liege. Another Column with Artillery having made its retreat by Namur, General Excelmans has the Order to push this evening six Squadrons on to Sart à Wallain, and three Squadrons on to Perwès. According to their report, if the mass of the Prussians is retiring on Wavre, I shall follow them in on Wavre, I shall follow them in that direction, so as to prevent them from reaching Brussels, and to keep them separated from Wellington.
"Si, au contraire, mes renseignemens prouvent que la principale force Prussienne a marche sur Perwès, je me dirigerai par cette ville à la poursuite de l'Ennemi. "If, on the contrary, my enquiries prove that the principal Prussian force has marched on Perwès, I shall proceed by that town in pursuit of the Enemy.
"Les Généraux Thielemann et Borstel faisaient partie de l'Armée que Votre Majesté a battue hier; ils étaient encore ce matin à 10 heures ici, et ont annoncé leurs avaient été mis hors de combat. Ils ont demande en partant que vingt mille hommes des les distances de Wavre, Perwès et Hannut. Blücher a été blessé légèrement au bras, ce qui ne l'a pas empêché de continuer à commander après s'être fait panser. Il n'a point passé par Gembloux. "Generals Thielemann and Borstel formed part of the Army They were still here at 10 o'clock this morning, and have announced that twenty thousand of their men have been disabled. They asked, in leaving, the distances of Wavre, Perwès, and Hannut. Blücher has been slightly wounded in the arm; which, however, has not hindered him from continuing to command after having his wound dressed. He has not passed by Gembloux.
"Je suis avec respect, de Votre Majesté,
"Sire, le fidèle sujet,
"Le Maréchal Comte de Grouchy." 		"I am with respect,
"Sire,
"The faithful subject of
"Your Majesty,
"Marshal Count de Grouchy."

Although the information conveyed in this despatch was incorrect on some points, and imperfect on others, inasmuch as it represented that Prussian Columns had retired upon Namur and Perwès, which was not the case, and gave no account of the Columns (First and Second Corps) which had retreated by Tilly and Gentinnes, still it was well calculated to satisfy Napoleon, that at least the spirit of his instructions had been understood by the Marshal. The latter had stated that he suspected a portion of the Prussian troops was proceeding to join Wellington, and that, should he ascertain, through his Cavalry detached to Sart à Wallain and Perwès, that the great mass of the Prussians was retiring upon Wavre, it was his intention to pursue them in that direction, "so as to prevent them from reaching Brussels, and to keep them separated from Wellington."

Four hours afterwards (that is, at two o'clock on the morning of the 18th) he sent off another despatch to the Emperor, reporting that he had decided on marching upon either Corbaix or Wavre.

The retreat of the Prussian Army, after its defeat at Ligny on the 16th of June, was conducted with great skill, and executed in very good order. By detaining Thielemann's Corps upon the Field of Battle until the morning of the 17th, ample security was afforded to the line of retreat by Gembloux; and by not withdrawing Bülow's Corps from that town until Thielemann drew near to it, the distance between the main bodies of these two Corps became so limited as to present the ready means of opposing their combined force to a vigorous pursuit should such be attempted.