Napoleon's other and far more disastrous mistake was taking for granted that Blucher had retreated on Namur, that is to say straight away from his ally. As a matter of fact the Prussians were retreating northwards on Wavre, true to the general agreement between Blucher and Wellington that they would co-operate as thoroughly as possible; and since that movement was ordered, news had come from Wellington that he was on the point of retreating on Waterloo, and would stand to fight there if assured of assistance from one Prussian corps. Napoleon had plenty of cavalry available, for Ligny was essentially an infantry battle, and certainly ought to have pushed cavalry along every road by which the Prussians could possibly have retreated. If they were gone eastwards, as he hoped and believed, all was plain sailing: if they were gone north, they might still unite with Wellington, and the game was by no means won. As it was, he contented himself with one reconnaissance along the Namur road, which confirmed him in his error by capturing a few stragglers, and so threw away the advantages gained already by his skilfully-devised plan of campaign. In the course of the morning of the 17th, Wellington withdrew his forces from Quatre-Bras, and retreated to the position at Waterloo which he had noted the year before, as an excellent one for a defensive battle to protect Brussels. In the afternoon heavy rain came on, which lasted all night, soaking the ground, seriously injuring the roads, and thus interfering with the march of the French, who followed at some distance. By nightfall the French were in front of Wellington's position; and both armies bivouacked on the wet ground, no very favourable preparation for the work of the morrow. Before moving from Ligny with the guard and Lobau's corps, to unite them to the troops under Ney, and with the whole body follow up the retreating English, Napoleon had given his orders for the pursuit of the defeated Prussians. Marshal Grouchy was put at the head of the two corps, Vandamme's and Gérard's, on which the stress of the fighting at Ligny had fallen, which with some cavalry amounted to about 33,000 men.