The First Corps d'Armée, which had from the beginning followed the Fourth as a Reserve, now advanced in pursuit of the Enemy by the direct road to Charleroi. The Light Cavalry at the head of the Column reached the passages of the Sambre at Châtelet, Charleroi, and Marchienne, without meeting any sort of opposition or impediment; nor did it perceive any thing of the Enemy on the other side of the river. The Corps halted for the night at Charleroi: having its Advanced Guard at Marchienne, and its Outposts occupying the line from Montigny by Louveral as far as Châtelet. Detachments from the Reserve Cavalry were sent in the direction of Fleurus, to secure the Corps from any molestation on the part of Grouchy; of whose proceedings nothing positive was then known at the Prussian Head Quarters.
It was not until nearly five o'clock in the evening of the 19th, that General Borcke, whose Brigade, the Ninth, was still in the vicinity of St Lambert, discovered the retreat of Grouchy's troops. He immediately communicated the fact to General Thielemann, who ordered him to cross the Dyle the next day (the 20th) and march upon Namur. The French Rear Guard of Gérard's Corps d'Armée continued to occupy Limale until nightfall. Thielemann remained posted, during the night of the 19th, at St Achtenrode; having his Advanced Guard at Ottenburg.
On the evening of the 18 th, Pirch received Orders to march from the Field of Waterloo with his Corps d'Armée (the Second) in the direction of Namur; for the purpose of turning Marshal Grouchy's Left Flank and intercepting his retreat upon the Sambre.
Pirch made this movement during the night, passing through Maransart, where he was joined by his Seventh Brigade; and crossing the Genappe rivulet at Bousseval, as also, subsequently, the Dyle, on his way to Mellery: which place he reached at eleven o'clock in the forenoon of the following day. His Corps was much divided on this occasion. He had with him the Sixth, Seventh, and Eighth Infantry Brigades, and twenty four Squadrons of Cavalry: but the Fifth Infantry Brigade, and the remaining fourteen Squadrons, were with that portion of the Prussian Army which was pursuing the Enemy along the high road to Charleroi. The Corps being greatly fatigued by the night march and its exertions on the previous day, Pirch ordered the troops to bivouac and to betake themselves to rest.
During this march, Lieutenant Colonel Sohr had pushed on with his Cavalry Brigade, as an Advanced Guard; and now he was required to gain intelligence concerning the Enemy's movements, and to seek a communication with Thielemann. He found the Defile of Mont St Guibert strongly occupied by the Enemy, but could obtain no information respecting Thielemann's Corps.
When it is considered how very near to Mellery Gérard's Corps d'Armée must have passed, in order to fall into the Namur road at Sombref; it seems extraordinary that Pirch, who reached that place at eleven o'clock in the forenoon of the 19th,—the same hour at which Grouchy, then beyond Wavre, received the first intimation of the defeat of Napoleon,—should have permitted Gérard to continue his retreat unmolested. His troops required rest, it is true; but had he maintained a good look out in the direction of Gembloux, he would, in all probability, after the lapse of a few hours, have been enabled to fulfil his instructions so far as to have completely intercepted the retreat of a considerable portion of Grouchy's Army. That part of the Enemy's force which Lieutenant Colonel Sohr observed at Mont St Guibert, was probably the Advanced Guard only of Gérard's Corps d'Armée; since its Rear Guard remained at the Bridge of Limale until nightfall. Taking all the circumstances into consideration, more especially the express object of the detached movement of the Second Prussian Corps d'Armée, it must be admitted that, on this occasion, there was a want of due vigilance on the part of General Pirch.
It was on the 19th, also, that Prince Blücher issued, whilst at Genappe, a Proclamation to his Army; in which he thanked the troops for their conduct during the recent struggle.
At daybreak of the 19th, that portion of the Duke of Wellington's Army which had fought the Battle of Waterloo, broke up from its bivouac, and began to move along the high road to Nivelles. Those troops which had been posted in front of Hal during the 18th, consisting of Stedman's Dutch-Belgian Division, Anthing's Dutch-Belgian Indian Brigade, and Colonel Estorff's Hanoverian Cavalry Brigade, under Prince Frederick of the Netherlands; as also of Johnstone's British Infantry Brigade, and Lyon's Hanoverian Infantry Brigade, under Lieutenant General Sir Charles Colville, were likewise directed to march upon Nivelles. The Army occupied Nivelles and the surrounding villages during the night of the 19th; in the course of which the Duke arrived from Brussels, and established his Head Quarters in the town.