At about 13:15, Napoleon saw the first columns of Prussians around the village of Lasne-Chapelle-Saint-Lambert, 4 to 5 miles (6.4 to 8.0 km) away from his right flank—about three hours march for an army. Napoleon's reaction was to have Marshal Soult send a message to Grouchy telling him to come towards the battlefield and attack the arriving Prussians. Grouchy, however, had been executing Napoleon's previous orders to follow the Prussians "with your sword against his back" towards Wavre, and was by then too far away to reach Waterloo.

Grouchy was advised by his subordinate, Gérard, to "march to the sound of the guns", but stuck to his orders and engaged the Prussian III Corps rear guard under the command of Lieutenant-General Baron von Thielmann at the Battle of Wavre. Moreover, Soult's letter ordering Grouchy to move quickly to join Napoleon and attack Bülow would not actually reach Grouchy until after 20:00.