On 16 June 1815, the French defeated the Prussians at the Battle of Ligny. The Prussians successfully disengaged and withdrew north to Wavre where they regrouped, and later advanced westward with three corps to attack the right flank of the French army at the Battle of Waterloo. The French were slow to exploit Ligny; Napoleon wasted the morning of 17 June with a late breakfast and touring the previous day's battlefield before organising a pursuit of the two Coalition armies. Napoleon and Marshal Michel Ney took the French reserves to pursue the Duke of Wellington's Anglo-allied army. Marshal Emmanuel de Grouchy was ordered to pursue and harry the Prussians and prevent them from regrouping.
Napoleon and Grouchy assumed that the Prussians were retreating towards Namur and Liège to take up a line on the river Meuse. On 17 June, Grouchy sent the bulk of his cavalry ranging in that direction as far as Perwez. From a 22:00 despatch to Napoleon, Grouchy still thought the Prussians were retreating north-east although by then he knew that two Prussian corps were heading north towards Wavre. From another despatch four hours later, Grouchy intended to advance to either Corbais or Wavre. However, at the end of 17 June his detachment was behind the Prussians, on the far side of the Dyle; Grouchy could neither prevent the Prussians at Wavre from moving to Waterloo, nor regroup with Napoleon on 18 June at Waterloo.
On the morning of 18 June, the remaining Prussians crossed the Dyle in and around Wavre and headed westwards towards Waterloo. Grouchy was at Sart-lez-Walhain when at about 11:30 he and his staff heard cannonades from the Battle of Waterloo, to the northwest; a local notary gave them an accurate location of the source of the sound. There were no direct roads from Grouchy to the battle, but there was a road to Wavre, away to the north-northwest. Grouchy moved on Wavre, arriving at 16:00. At the Battle of Wavre, the French defeated the Prussian rearguard, the Prussian III Corps commanded by Johann von Thielemann, which had been about to leave for Waterloo.
The Prussian advance to Waterloo was impeded by swollen streams, which turned their valleys into muddy swamps, in particular the small river Lasne close to Saint-Lambert. Napoleon neglected to screen his right flank and failed to detect the Prussian's approach. At around 17:00 on 18 June, the Prussian vanguard started to arrive at Waterloo in strength from the Wood of Paris; shortly after they were attacking the right flank of Napoleon's army.
Ligny to Wavre, 17 June Prussian retreat from Ligny
After the Battle of Ligny, Zieten's Prussian I Corps and Pirch I's II Corps retired to Tilly and Gentinnes.
On the night of 16 June, Prussian headquarters ordered the army to fall back to Wavre instead of falling back along lines of communication toward Prussia; by doing so, Field Marshal Gebhard Leberecht von Blücher retained the option of linking-up with the Anglo-allied army. I and II Corps were ordered to Bierges and Saint-Anne. Thielemann's III and Bülow's IV Corps at Gembloux and received the order on the morning of 17 June to move and bivouac at and near Wavre.
I and II Corps passed through Mont-Saint-Guibert. II Corps deployed as rearguard in a defile behind the settlement. I Corps arrived at Wavre around noon, crossed the Dyle, and took up positions at Bierges. II Corps followed, but took up position on the right bank of the Dyle, between Sainte Anne and Aisémont.
A cavalry brigade with half a horse battery, commanded by Lieutenant Colonel Eston von Sohr, from II Corps acted as rearguard for I and II Corps. It was ordered to take up a concealed position between Tilly and Gentinnes and observe French movements, and fall back to the defile at Mont-Saint-Guibert when engaged.
Also that night, Colonel von Röhl, superintendent of the army's Ordnance Department, set out for Wavre to organize any artillery arriving there for action. His aide-de-camp went to Gembloux to move the army's reserve ammunition wagons to Wavre. Additional precautions were taken. Urgent orders for ammunition were sent to Maastricht, Cologne, Wesel, and Münster. Orders went to Liège to move the siege train to Maastricht, and to destroy the iron foundry at the arsenal.
General Friedrich Wilhelm von Jagow's detachment had held Byre during the night. At first light on 17 June, it began moving to Gembloux by way of Sombreffe, and arrived before III Corps. After receiving the retreat order, Jagow directed the detachment's units back to their brigades.
Prussian III Corps joins IV Corps at Gembloux Prussian III Corps remained in place for much of 16 June, which covered the retreat through Gembloux.
Blücher was recovering from his fall at the Battle of Ligny. It was Gneisenau, Blücher's chief-of-staff, who gave III Corps the option of retreating by Tilly or Gembloux. Thielemann chose Gembloux, as he knew that the French held Saint-Amand, Ligny, and the Ligny battlefield up to Sombreffe.
The widely dispersed III Corps regrouped during the night of 16 June; the manoeuvre was protracted. It was not until 02:00 on 17 June that the head of the column, made of the reserve artillery, reached the Nivelles–Namur/Fleurus–Gembloux crossroad and the Gembloux road. The corps' main body reached Gembloux at 06:00.
III Corps' rearguard—Borcke's 8th Infantry Brigade and Hobe's III Corps Cavalry (the reserve cavalry)—took up position along the Namur road, facing south down the Fleurus chaussée (high road) toward the French. The rearguard withdrew at 04:00.
IV Corps' vanguard reached Baudecet by the old Roman at nightfall on 16 June. Bülow learned of the Battle of Ligny, and ordered brigades posted at intervals along the road. Hake's 13th Brigade was bivouacked to the rear, near where the road intersected with the Namur-Louvain road. In the morning, IV Corps was posted about to the rear of Grembloux on the Roman road.
As III Corps approached Gembloux, it learned of IV Corps' position. Major Karl von Weyrach, Blücher's aide-de-camp, and who had accompanied III Corps through the night, left to report the status of both corps to army headquarters; he found it at Mellery. III Corps halted on the other side of the town. Neither Thielemann nor Bülow knew where I and II Corps were. Writing to Bülow, Thielemann said that he had no new orders from Blücher, but believed the army was retreating to Saint-Trond. Furthermore, III Corps had not been followed by the French, but he had heard distant firing on the right, which he concluded was connected with Duke of Wellington's Anglo-allied Army.
With two corps available, the Prussians could contest a French pursuit toward Gembloux.
Cavalry skirmish along the Namur road The Prussians were too far away to easily follow by the time the French organized a pursuit. The French detached Pajol and the 4th Cavalry Division of his light Cavalry Corps to pursue. The division, led by Pierre Soult, consisted of Clary's 1st, Blot's 4th, and Liégeard's 5th Hussars; they struck down the Namur road. They were supported by Lieutenant General Baron François Antoine Teste's 21st Infantry Division of Lobau's VI Corps, which took up position on the heights above Mazy.
4th Cavalry Division quickly encountered a Prussian force composed of Horse Battery No. 14 and a squadron of the 7th Uhlans. The horse battery, from II Corps, had retired during the Battle of Ligny after running out of ammunition but had not found the reserve ammunition wagons; it had neither returned to II Corps or withdrawn with III Corps, instead "uselessly driving first in one direction, and then in another". The Uhlans, from III Corps, had been posted at Onoz and had not received recall orders. The French captured all of the Prussian guns, with the Uhlans losing 30 men before escaping. The skirmish gave the French the impression that the Prussians were retreating toward Namur.
Prussian III Corps and IV Corps retreat to Wavre At about 09:30, Weyrach arrived at IV Corps headquarters with orders from army headquarters for IV Corps to move to Dion-le-Mont, near Wavre, by Walhain and The orders also defined the corps' rearguard. The main body of the rearguard—ultimately 14th Brigade—was to be posted at Vieux-Sart. Furthermore, a detachment of a regiment of cavalry, two battalions of infantry, and two guns of horse artillery was to be sent to Mont-Saint-Guibert to reinforce, and then cover the withdrawal of, I and II Corps' rearguard; this detachment was commanded by Lieutenant Colonel Ledebur and composed of the 10th Hussars, the fusilier battalions of the 11th Regiment of Infantry and 1st Regiment of Pomeranian Landwehr, and two guns from Horse Battery No. 12.
The main body of the IV Corps moved directly to Dion-le-Mont. It took up position on the height near that town, near the intersection of the roads leading to Louvain, Wavre, and Gembloux.
III Corps began moving at 14:00; it arrived at Wavre late in the evening and took up positions at Bawette. General Borcke's 9th Infantry Brigade and Colonel Count Lottum's Cavalry Brigade remained on the right bank of the Dyle. Other units rejoined the main body at Wavre: Colonel Marwitz' Cavalry Brigade which had withdrawn through Tilly, the 2nd Battalion of the 3rd Kurmark Landwehr and the two squadrons of the 6th Kurmark Landwehr Cavalry from Dinant, and the survivors of the 7th Uhlans squadron from Onoz. The corps' two squadrons from the 9th Hussars had not yet arrived from Ciney.
Prussian headquarters retreats to Wavre Prussian army headquarters relocated to Wavre early on 17 June. Blücher was still recovering from his injuries and remained bedridden for the rest of the day.
Late in the morning, Lieutenant made a round trip as messenger between the Prussians and the Anglo-allies. He notified the Anglo-allies that the Prussians were retreating to Wavre, and then returned with a message from Wellington for Blücher. Wellington intended to retreat to Waterloo, and requested the support of two Prussian corps so he could give battle. Blücher did not reply immediately. The request was viewed favourably but the Prussian army was still regrouping on Wavre and Blücher did not yet know what he could promise. It was not known if IV Corps would join the army on 17 June. I and II Corps needed ammunition, but its supply had been directed to Grembloux. For the moment, the Prussians maintained their position near the Dyle, with VI Corps at Mont-Saint-Guibert as vanguard.
During the day, Major Falkenhausen conducted reconnaissance toward Genappe from . He observed the French advance along the Genappe-Brussels high road from woods beyond Céroux. Prussian patrols were sent toward Lasne, Couture-Saint-Germain, and , to observe the defiles along the streams of the Lasne.
French reconnaissance and inactivity The Prussian retreat, in good order and along routes where only minor roads (as opposed to high roads) were available, was met by French inactivity. At dawn on 17 June, French vedettes were within of Prussian III Corps' rearguard. The French did not conduct patrols, and missed the opportunity to observe the direction of the rearguard's withdrawal after sunrise. Pajol's detachment, perhaps suspecting Namur was not the direction of the Prussian retreat, went to Saint-Denis where it regrouped with the 21st Infantry Division. A brigade from Exelmans' Heavy Cavalry Corps was sent to reinforce Pajol's detachment; based on updated intelligence, it redirected en route to Gembloux and discovered traces of the Prussian retreat.