The full importance of this choice will, after the constant reiteration of it in the preceding pages, be clear to the reader. A retreat towards the east, and upon the line of communications which fed the Prussian army, would have had these two effects: First, it would have involved in the retirement that fresh Fourth Army Corps under Bulow which had not yet come into action, and which numbered no less than 32,000 men. For it lay to the east of the battlefield. In other words, that army corps would have been wasted, and the whole of the Prussian forces would have been forced out of the remainder of the campaign. Secondly, it would have finally separated Blucher and his Prussians from Wellington's command. The Duke, with his western half of the allied forces, would have had to stand up alone to the mass of Napoleon's army, which would, after the defeat of the Prussians at Ligny, naturally turn to the task of defeating the English General. Now the fact of capital importance upon which the reader must concentrate if he is to grasp the issue of the campaign is the fact that the French staff fell into an error as to the true direction of the Prussian retreat. Napoleon, Soult, and all the heads of the French army were convinced that the Prussian retreat _was_ being made by that eastern road. As a fact, the Prussians, under the cover of darkness, had retired _not_ east but north. The defeated army corps, the First, Second, and Third, did not fall back upon the fresh and unused Fourth Corps; they left it unhampered to march northward also; and all during the darkness the Prussian forces, as a whole, were marching in roughly parallel columns upon Wavre and its neighbourhood. It was this escape to the north instead of the east that made it possible for the Prussians to effect their junction with Wellington upon the day of Waterloo; but it must not be imagined that this supremely fortunate decision to abandon the field of their defeat at Ligny in a northerly rather than an easterly direction was at first deliberately conceived by the Prussians with the particular object of effecting a junction with Wellington later on. In the first place, the Prussians had no idea what line Wellington's retreat would take. They knew that he was particularly anxious about his communications with the sea, and quite as likely to move westward as northward when Napoleon should come against him.