On the 21st, Grouchy marched from Dinant to Phillippeville, but Pirch I. did not pursue. His Corps was required elsewhere, to blockade some of the fortresses which barred the line of advance of Blucher’s army. Grouchy might have retired through Givet and down the valley of the Meuse, instead of risking the more dangerous road to Phillippeville. But his aim was to draw near to any body of troops which were left from Napoleon’s army, and to avoid marching down the narrow defile of the Meuse valley where he would be liable to an attack in flank or in rear, under great disadvantages. Zieten was at Beaumont on 20th June, 12 miles from Phillippeville, but he had marched at daybreak on the 21st. Pirch, marching to Thuin on the 21st, was moving parallel to Grouchy, but the latter’s march was not hindered. Four French fortresses barred Blucher’s advance--Landrécies, Maubeuge, Avesnes and Rocroi. It was necessary to reduce these before any further advance on Paris was made; hence Grouchy was able to retreat unmolested for the greater part of his movement. On the 22nd he reached Rocroi; and Mezières on the 23rd. His force constituted an important menace to the left flank of the Prussian army; and Blucher was thus obliged to detach several parties of cavalry to watch the French movements. Zieten took Avesnes on the 21st, and Grouchy’s march from Phillippeville to Rocroi was in danger; but his strength was not accurately known at the Prussian headquarters, and Blucher was anxious to push on to Paris. The fall of Paris was expected to put an end to the French resistance. The capture of Avesnes relieved Blucher of the danger which threatened his army if he advanced, and it also gave him an advanced depôt for his supplies.