Victor Hugo
Vol. II, Book I, Chap. XI: “A Bad Guide to Napoleon; A Good Guide to Bulow”
(VOLUME II: COSETTE; BOOK I: WATERLOO)

The painful surprise of Napoleon is well known. Grouchy hoped for, Blucher arriving. Death instead of life.
Fate has these turns; the throne of the world was expected; it was Saint Helena that was seen.
If the little shepherd who served as guide to Bulow, Blucher's lieutenant, had advised him to debouch from the forest above Frischemont, instead of below Plancenoit, the form of the nineteenth century might, perhaps, have been different. Napoleon would have won the battle of Waterloo. By any other route than that below Plancenoit, the Prussian army would have come out upon a ravine impassable for artillery, and Bulow would not have arrived.
Now the Prussian general, Muffling, declares that one hour's delay, and Blucher would not have found Wellington on his feet. "The battle was lost."
It was time that Bulow should arrive, as will be seen. He had, moreover, been very much delayed. He had bivouacked at Dion-le-Mont, and had set out at daybreak; but the roads were impassable, and his divisions stuck fast in the mire. The ruts were up to the hubs of the cannons. Moreover, he had been obliged to pass the Dyle on the narrow bridge of Wavre; the street leading to the bridge had been fired by the French, so the caissons and ammunition-wagons could not pass between two rows of burning houses, and had been obliged to wait until the conflagration was extinguished. It was mid-day before Bulow's vanguard had been able to reach Chapelle-Saint-Lambert.
Had the action been begun two hours earlier, it would have been over at four o'clock, and Blucher would have fallen on the battle won by Napoleon. Such are these immense risks proportioned to an infinite which we cannot comprehend.
The Emperor had been the first, as early as mid-day, to descry with his field-glass, on the extreme horizon, something which had attracted his attention. He had said, "I see yonder a cloud, which seems to me to be troops." Then he asked the Duc de Dalmatie, "Soult, what do you see in the direction of Chapelle-Saint-Lambert?" The marshal, levelling his glass, answered, "Four or five thousand men, Sire; evidently Grouchy." But it remained motionless in the mist. All the glasses of the staff had studied "the cloud" pointed out by the Emperor. Some said: "It is trees." The truth is, that the cloud did not move. The Emperor detached Domon's division of light cavalry to reconnoitre in that quarter.
Bulow had not moved, in fact. His vanguard was very feeble, and could accomplish nothing. He was obliged to wait for the body of the army corps, and he had received orders to concentrate his forces before entering into line; but at five o'clock, perceiving Wellington's peril, Blucher ordered Bulow to attack, and uttered these remarkable words: "We must give air to the English army."
A little later, the divisions of Losthin, Hiller, Hacke, and Ryssel deployed before Lobau's corps, the cavalry of Prince William of Prussia debouched from the forest of Paris, Plancenoit was in flames, and the Prussian cannon-balls began to rain even upon the ranks of the guard in reserve behind Napoleon.