Thielemann only heard of the French retreat towards 6 P.M. on the 19th, and his intelligence came through General Borcke, who discovered Grouchy’s movement, from St Lambert. Pajol had a rear-guard still in front of Thielemann, and as the latter’s troops were tired with their recent exertions, the Prussians postponed their pursuit until the next day, the 20th, when Borcke was ordered to march at daybreak with the Ninth Division from St Lambert, across the Dyle, and towards Namur.
At daybreak on the 20th, Grouchy’s rear-guard left Gembloux and marched on Namur by St Denis and La Falize. His infantry left Temploux about 9 A.M. Gérard’s Corps was intended to lead, Vandamme’s Corps covering the retreat of the Fourth, but Vandamme upset the arrangements by betaking himself over-night to Namur, leaving no instructions behind him for his Divisional generals. Consequently, the Divisions of the Third Corps moved off by themselves, early in the morning, and Gérard’s Corps, which was carrying the wounded with it, was left uncovered. A short distance beyond Temploux, the column was attacked by Prussian cavalry which had been sent off in pursuit by Thielemann at daybreak that morning. And at the same time, more cavalry were seen coming against the rear, along the Nivelles-Namur road. This was the cavalry heading Pirch’s Corps, which had marched from Mellery to Sombreffe. Gérard’s column had now stumbled on Vandamme’s rear-guard, posted 3 miles outside Namur; and Vandamme himself coming out from Namur, Grouchy ordered him to clear the road for the Fourth Corps, and cover its march with his rear-guard.
Thielemann’s Cavalry, accompanied by a battery of horse artillery, had come on at a great pace, and were almost too exhausted to attack the French with any vigour; but they managed to drive back the enemy’s cavalry and capture three guns. Further attack on the French rear-guard was left to Pirch’s Corps, which was now hurrying up.
The French retreated through Namur, after being well treated by the inhabitants (who supplied them with food, transport, and boats), and leaving Teste’s Division with eight guns in defence of the town against Pirch’s Corps. The remainder of Grouchy’s army crossed the Sambre by the Namur bridge and marched on Dinant by the valley of the Meuse.