Napoleon took the reserve of the Army of the North, and reunited his forces with those of Ney to pursue Wellington's army, after he ordered Marshal Grouchy to take the right wing of the Army of the North and stop the Prussians re-grouping. In the first of a series of miscalculations, both Grouchy and Napoleon failed to realise that the Prussian forces were already reorganised and were assembling at the village of Wavre. The French army did nothing to stop a rather leisurely retreat that took place throughout the night and into the early morning by the Prussians. As the 4th, 1st, and 2nd Prussian Corps marched through the town towards Waterloo the 3rd Prussian Corps took up blocking positions across the river, and although Grouchy engaged and defeated the Prussian rearguard under the command of Lt-Gen von Thielmann in the Battle of Wavre (18–19 June) it was 12 hours too late. In the end, 17,000 Prussians had kept 33,000 badly needed French reinforcements off the field.