Waterloo by Thomas E. Watson — Read

While it was not disorganized or demoralized, Blücher's army was in great peril. Two of his army corps were concentrated at Wavre, one was at Gembloux, and the fourth at Wandesett. Had the French been vigilant, these separated corps might have been overwhelmed in detail. Through the carelessness of videttes, the lack of enterprise in the leaders of reconnoitering parties and the unpardonable neglect of General Exelmans, neither Napoleon nor Grouchy was informed of the movement of the Prussian corps.

After Grouchy was given command of 33,000 troops to pursue the Prussians, the delays in starting, the slowness of the march, the lack of harmony between Grouchy and his two lieutenants, Vandamme and Gérard, made the "pursuit" the most futile on record.