withdrew to north west to Wavre where they reorganised. Leaving one corps in Wavre as a blocking rearguard, the three other corps advanced westward to attack the right flank of the French army in front of Waterloo. Both Napoleon and Grouchy assumed that the Prussians were retreating towards Namur and Liège, with a view to occupy the line of the river Meuse, and so during 17 June Grouchy sent the bulk of his cavalry ranging in that direction as far as Perwez. In his despatch to Napoleon written at 22:00 he was still thought that most of the Prussian army was retreating north-east, although by then he realised that two Prussian corps were heading north towards Wavre. In a second dispatch written four hours later he informed Napoleon that he now intended to advance either on Corbais or Wavre. The problem for the French was that by the end of 17 June, most of Grouchy's detachment was now behind the Prussians, on the far side of the Dyle. This meant that they were incapable of preventing the Prussians moving from Wavre towards Waterloo and too far away themselves to go directly to the aid of Napoleon on 18 June should Wellington turn and fight south of Brussels. Upon receiving the news of Blücher's defeat, Wellington organised the retreat of the Anglo-allied army to a place he had identified a year before as the best place in front of Brussels for him to be able to employ his reverse slope tactics when fighting a major battle: Mont-Saint-Jean escarpment close to the village of Waterloo. Aided by thunderstorms and torrential rain, Wellington's army successfully extricated itself from Quatre Bras and passed through the defile of Genappe. The infantry marched ahead and were screened by a large cavalry rearguard. The French harried Wellington's army, and there was a cavalry action at Genappe. However the French were unable to inflict any substantial casualties before night fell and Wellington's men were ensconced in bivouacs on the plain of Mont-Saint-Jean. Waterloo (18 June) It was at Waterloo on 18 June 1815 that the decisive battle of the campaign took place. The start of the battle was delayed for several hours as Napoleon waited until the ground had dried from the previous night's rain. By late afternoon the French army had not succeeded in driving Wellington's forces from the rise on which they stood. Once the Prussians