Napoleon rightly estimated that he had surprised the foe, though he exaggerated the extent of that surprise. He thought it possible that this body to the left, under Ney, might push on to Brussels itself, and in any case could easily deal with the small and unprepared forces which it might meet upon the way. Its function in any case, whether resistance proved slight or formidable, was to hold the forces of Wellington back from effecting a junction with Blucher and the Prussians. Meanwhile, the right-hand body, under Grouchy, was to fall upon the extremity of the Prussian line and overwhelm it. [Illustration] Such an action against the head of the long Prussian cordon could lead, as the Emperor thought, to but one of two results: either the great majority of the Prussian force, coming up to retrieve this first disaster, would be defeated in detail as it came; or, more probably, finding itself cut off from all aid on the part of Wellington's forces to the west and its head crushed, the long Prussian line would roll up backwards upon its communications towards the east, whence it had come. In either case the prime object of Napoleon's sudden move would have been achieved; and, with the body upon the left, under Ney, pushing up the Brussels road, the body upon the right, under Grouchy, pushing back the head of the Prussian line eastward, the two halves of the Allies would be separated altogether, and could later be dealt with, each in turn. The capital disadvantage under which Napoleon suffered--the fact that he had little more than half as many men as his combined enemies--would be neutralised, because he would, after the separation of those enemies into two bodies, be free to deal with either at his choice. Their communications came from diametrically opposite directions,[5] and, as the plan of each depended upon the co-operation of the other, their separation would leave them confused and without a scheme. Napoleon in all this exaggerated the facility of the task before him; but before we go into that, it is essential that the reader should grasp a certain character in all military affairs, to misunderstand which is to misread the history of armies. _This characteristic is the necessary uncertainty under which every commander lies as to the disposition, the number, the order, and the information of his opponents._ It is a _necessary_ characteristic in all warfare, because it is a prime duty in the conduct of war to conceal from your enemy your numbers, your dispositions, and the extent of your information. It is a duty which every commander will always fulfil to his best ability.