The difficulty with this reasoning, as it seems to us, lies in the fact that it was not that he might be strong enough to defeat Wellington that Napoleon needed to have all his army on the field of Waterloo, but in order to prevent his being overwhelmed by the union of the army of Blücher to that of Wellington. With the army of Wellington the force Napoleon had at Waterloo was abundantly able to cope; but against the two armies of Wellington and Blücher united, he was certain to succumb. Hence, inasmuch as he, at 1 P.M. of the 17th, had changed his mind as to the Prussian retreat, had come to regard as very questionable the view which until after 12 o’clock of that day he certainly had entertained, that the Prussians were separating themselves definitely from the English, and had recognized the possibility of their uniting with the English and even joining with them in a battle to be fought for the defence of Brussels, he should have changed his dispositions accordingly. He should have left a comparatively small force,—perhaps of cavalry only,—to follow up and find out about the Prussians, and should have taken all his army-corps with him, because if the Prussians should attempt to take part in the battle he would be sure to need every man he could muster.[641] 3. It seems for many writers to be well-nigh impossible to treat separately of the conduct of Marshal Grouchy. This may require some effort of mind; but it surely is worth while to make it.[642] Suppose we are of opinion that Napoleon made a mistake in the first place in detaching him with his 33,000 men; that he was furthermore inexcusable in not advising him on the night of the 17th and 18th of the most important fact that the two armies were confronting each other at Mont St. Jean,—in not sending him the information which Domon had picked up as to the retreat of a part of the Prussians towards Wavre,—and, finally, in not directing him in precise terms, that, if he should find that the Prussians were retiring on Wavre, he should cross the Dyle at Moustier and Ottignies, and operate in connection with the main army. Suppose, we say, we find all this to be true. We have yet to pass upon Grouchy’s conduct. The Emperor’s faults cannot excuse Grouchy’s faults, nor can his faults excuse those of the Emperor.[643] The object of military criticism is not to see which officer made the most mistakes. 4. As to the proper course for Marshal Grouchy to take after he became satisfied (as he did about daybreak of the 18th) that the Prussians were retreating on Wavre, we have seen that the principal continental critics,—Jomini, Clausewitz and Charras,—are of opinion that he should at once have manœuvred to his left and crossed the Dyle, so as to get in communication with the main army under the Emperor.