Napoleon received Marshal Grouchy’s letter, dated Gembloux, 10 P.M. of June 17th, about 2 A.M. of the 18th, at the Caillou House, on the Brussels turnpike, where he passed the night of the 17th. A close examination of it might have raised a suspicion in his mind that Grouchy did not thoroughly comprehend his task, and that he might possibly fail to take the right course, if the emergency, which he had in his letter represented as not unlikely to occur, should actually confront him. He had said, that, if he found that the mass of the Prussians were retiring on Wavre, he “would follow them in that direction, in order that they might not be able to gain Brussels, and to separate them from Wellington,” but if they were retiring on Perwez, that he would direct himself on that city. We have pointed out above that it was clearly impossible for him to prevent the Prussians from getting to Brussels. He was thirty miles from Brussels,—the Prussians less than twenty,—and they were directly between him and Brussels. And as for separating the Prussians at Wavre from Wellington, while Grouchy must of course follow them in the direction of Wavre as distinguished from that of Perwez, yet the only thing really open to him was to cross the Dyle at once by the bridges of Moustier and Ottignies and then to act in close connection with the main French army,—to stand between it and the Prussians, and ward off the danger as best he might. This could be done; but this was all that could be done. It was not to be expected that an attack upon the Prussian rearguard at Wavre,—which was the only other thing that Grouchy could do,—however vigorously made, could have the result of detaining their whole army. But, in Grouchy’s letter, a movement on his part to rejoin or to approach the main army by crossing the Dyle, in case he found that the Prussians were massing at Wavre, was not even mentioned.