The retreat of the Prussians was not interrupted, and was seemingly unnoticed, by the French.
Crucially, they retreated not to the east, along their lines of communication and away from Wellington, but northwards, parallel to Wellington's line of march and still within supporting distance, and remained throughout in communication with Wellington.
They regrouped south of Wavre, around 13 kilometres (8 mi) march to the east of Waterloo, from whence they proceeded to Waterloo on 18 June.

Wellington anticipated that Napoleon would then come against him at Quatre Bras, so he  spent 17 June retreating northwards to a defensive position he had personally reconnoitred the previous year at Mont-Saint-Jean a low ridge south of the village of Waterloo and the Forest of Soignes.
Napoleon, with the reserve and the right-wing of the Army of the North, made a late start on 17 June and joined Ney at Quatre Bras at 13:00 to attack Wellington's army, but found only the rearguard of cavalry remaining on the site.
The French pursued Wellington to Waterloo but managed only to defeat a small cavalry rearguard in an action at Genappe before torrential rain set in for the night.
By the end of 17 June, Wellington's army had arrived at its position at Waterloo, with the main body of Napoleon's army following.
It was at Waterloo on 18 June 1815 that the decisive battle of the campaign took place.

Before leaving Ligny, Napoleon gave Grouchy 33,000 men and orders to follow up the retreating Prussians.
A late start, uncertainty about the direction the Prussians had taken, and the vagueness of the orders given to Grouchy meant that he was too late to prevent the Prussian army reaching Wavre, from where it could march to support Wellington.
After the French defeat at Waterloo, only Grouchy managed to retreat in good order to France with his force of nearly 30,000 organised French soldiers with their artillery.
However, this army was not strong enough to resist the combined coalition forces.
Napoleon announced his abdication on 24 June 1815 and finally surrendered on 15 July.
The retreat of the Prussians was not interrupted, and was seemingly unnoticed, by the French. Crucially, they retreated not to the east, along their lines of communication and away from Wellington, but northwards, parallel to Wellington's line of march and still within supporting distance, and remained throughout in communication with Wellington. They regrouped south of Wavre, around 13 kilometres (8 mi) march to the east of Waterloo, from whence they proceeded to Waterloo on 18 June.
Wellington anticipated that Napoleon would then come against him at Quatre Bras, so he  spent 17 June retreating northwards to a defensive position he had personally reconnoitred the previous year at Mont-Saint-Jean a low ridge south of the village of Waterloo and the Forest of Soignes. Napoleon, with the reserve and the right-wing of the Army of the North, made a late start on 17 June and joined Ney at Quatre Bras at 13:00 to attack Wellington's army, but found only the rearguard of cavalry remaining on the site. The French pursued Wellington to Waterloo but managed only to defeat a small cavalry rearguard in an action at Genappe before torrential rain set in for the night. By the end of 17 June, Wellington's army had arrived at its position at Waterloo, with the main body of Napoleon's army following. It was at Waterloo on 18 June 1815 that the decisive battle of the campaign took place.
Before leaving Ligny, Napoleon gave Grouchy 33,000 men and orders to follow up the retreating Prussians. A late start, uncertainty about the direction the Prussians had taken, and the vagueness of the orders given to Grouchy meant that he was too late to prevent the Prussian army reaching Wavre, from where it could march to support Wellington. After the French defeat at Waterloo, only Grouchy managed to retreat in good order to France with his force of nearly 30,000 organised French soldiers with their artillery. However, this army was not strong enough to resist the combined coalition forces. Napoleon announced his abdication on 24 June 1815 and finally surrendered on 15 July.
Battle of Ligny