It is at this stage advisable to consider why Napoleon had chosen the crossing of the Sambre at Charleroi and the sites immediately to the north on the left bank of that river as the point where he would strike at the long line of the Allies. Many considerations converged to impose this line of advance upon Napoleon. In the first place, it was his task to cut the line of the Allies in two at the point where the extremity of one army, the Prussian, touched upon the extremity of the other, that of the Duke of Wellington. This point lay due north of the river-crossing he had chosen. Again, the main road to Brussels was barred by the fortress of Mons, which, though not formidable, had been put in some sort of state of defence. Again, as a glance at the accompanying map will show, the Prussian half of the allied line was drawn somewhat in front of the other half; and if Napoleon were to attack the enemy in detail, he must strike at the Prussians first. Finally, the line Maubeuge-Philippeville, upon which he concentrated his front, was, upon the whole, the most central position in the long line of his frontier troops, which stretched from Metz to the neighbourhood of the Straits of Dover. Being the most central point, not only with regard to these two extremities, but also with regard to distant Paris, it was the point upon which his concentration could most rapidly be effected. [Illustration] This, then, was the position upon the night of the 14th. The three great bodies of French troops (much the largest of which was that in the centre) to march at dawn, the light cavalry moving as early as half past two, ahead of the centre, the whole body of which was to march on Charleroi. The left, that is the First and Second Corps, to cross the Sambre at Thuin, the Abbaye d'Aulne, and Marchiennes. (There were bridges at all three places.) The right or Fourth Corps was also to march on Charleroi.[3] Napoleon intended to be over the river with all his men by the afternoon of the 15th, but, as we shall see, this "bunching" of fully half the advance upon one crossing place caused, not a fatal, but a prejudicial delay. Among other elements in this false calculation was an apparent error on the part of Soult, who blundered in some way which kept the Third Corps with the centre instead of relieving the pressure by sending it over with the Fourth to cross, under the revised instructions, by Châtelet. [Illustration: Disposition of the Four Prussian Corps on June 15th, 1815.] At dawn, then, the whole front of the French army was moving. It was the dawn of Thursday the 15th of June. By sunset of Sunday all was to be decided. * * * * * At this point it is essential to grasp the general scheme of the operations which are about to follow. Put in its simplest elements and graphically, the whole business began in some such form as is presented in the accompanying sketch map. [Illustration]