When Napoleon and Wellington stood at length face to face on the night of June 17, both were naturally anxious about the morrow, but in singularly different ways. The emperor, justly believing that his army was the better of the two, was only afraid that Wellington might yet rob him of victory by decamping in the dark; and this fear haunted him so obstinately that in the middle of the night, and again at daybreak, he rode out to satisfy his own eyes that the English army was still in position. Strangely enough he does not seem to have been at all apprehensive of the contingency which in fact happened to his ruin: he still supposed the Prussian army out of reach. On the morning of the 18th he sent a despatch to Grouchy, which accepted as the right thing to be done that general's intention of marching on Wavre. Not till eleven a.m., when the battle was on the point of beginning, did he send a regiment[78] of cavalry towards the bridges by which Grouchy would have had to cross the Dyle, as if that were a chance worth taking account of. But he took no steps to secure Grouchy's coming, till the first Prussian troops were in sight,[79] as he certainly would have done had he seriously feared Blucher taking him in flank. And this reduplication of the grievous mistake he had made on the field of Ligny was absolutely fatal.
Wellington on the other hand had fully made up his mind to fight, in spite of the great risk he ran of being overwhelmed before the Prussians could reach him, assuming always that Blucher would come to his assistance. How great that risk was, it is a little hard to realise after the event. He could have no knowledge of the amount of Grouchy's army, though he may well have guessed that Napoleon had detached some troops to observe the Prussians. For all he knew the French might be outnumbering him considerably, the more so as he still deemed it necessary to guard his right by leaving a large force at Hal, some ten or twelve miles off. Napoleon made a miscalculation, as most critics think, in giving Grouchy so large a force. If Grouchy was merely to follow up the Prussians retreating eastwards, less would suffice: if the emperor was to encounter at Waterloo the allied armies united, he needed every man. And Wellington made a similar miscalculation in leaving 18,000 men at Hal: if he was beaten at Waterloo, there would be no longer a flank to guard; if he was successful, a French detachment sent to turn his flank would have great difficulty in escaping destruction. Even on the assumption that Wellington knew the strength of the army facing him, he was outmatched so long as he stood alone: and what was his security for being supported? It has been said already that he and Blucher acted cordially together, but there was not, and could not be, a common plan of action worked out in detail. The very fact that they had to await attack, wherever Napoleon might assail them, rendered any such elaborate concert impossible. Moreover Wellington was doubtless aware that Gneisenau would be slow to take a course that endangered the Prussians. If after all the Prussians were not coming, there was yet time to retreat from an untenable position. Under the pressure of this anxiety, Wellington is said to have spent a great part of the night in riding over to Wavre and back, in order to see Blucher and make sure. Whatever passed at that interview,[80] the duke was satisfied, and on the fateful morning of the 18th, his troops stood to their arms in the stations already assigned to them.