By neglecting to move the whole of his division upon Quatre-Bras early in the morning, Ney failed to cut off the means of junction between the Prussian and English armies; and by not sending the detachment to attack the Prussians in the rear at Ligny, it now appears that the whole Prussian army was saved from being destroyed, or made prisoners, before it could receive the full support which had been promised by the Duke of Wellington. The latter intended to advance on Quatre-Bras at 2 o'clock, and debouch on St. Amand at 4 p. m. Ney, however, did an important act in checking the advance of five or six divisions of the main army during the rest of the day while the battle of Ligny was decided, and in this repaired, in a measure, his various faults committed on the 16th.

The French bivouacked, on the night of the 16th, on the battlefield of Ligny, with the exception of Grouchy's division, which encamped at Sombref. The Duke of Wellington passed the night at Quatre-Bras,—his army gradually joining him till the morning of the 17th,—when they amounted to 50,000 men. The victory acquired by Napoleon at Ligny did not fulfill his expectations. "If Marshal Ney had attacked the British with his united forces," said the Emperor, "they must inevitably have been crushed; after which, he might have given the Prussians a conclusive blow; but, even if after neglecting that first step he had not committed a second, in impeding the movement of Count d' Erlon, the appearance of the first corps would have curtailed Blucher's resistance, and secured his overthrow without a possibility of doubt; then his entire army must have been captured or annihilated."

Ney was now ordered to advance on Quatre-Bras at daybreak, and attack the British rear-guard, while Count Lobau was to proceed along the causeway of Namur, and take the British in flank. General Pajol, at daybreak, also went in pursuit of the Prussians under Blucher. He was supported by Grouchy, with Excelmans' cavalry, and the third and fourth corps of infantry, amounting in all to about 32,000 men. Grouchy was ordered by the Emperor to "above all things, pursue the Prussians briskly, and keep up a communication with me to the left" so as to rejoin the main army whenever required.

Napoleon rode over the field of battle at Ligny, and directed every assistance be given to the wounded. He then hurried to the support of Ney's attack on Quatre-Bras. He learned that it was still held by the British, and that Ney had not made the attack. He reproached Ney on meeting him, and the marshal excused his delay by declaring he believed the whole British army was there. This, however, was not the case.

The Duke of Wellington, who intended a junction with the Prussians at Quatre-Bras,—but had been frustrated by their disastrous defeat at Ligny,—now ordered a retreat on Brussels, leaving the Earl of Uxbridge, with his cavalry, as a rear-guard. Napoleon directed Count Lobau's division to advance, and the British cavalry then began to retire in battle-array. The French army moved forward in pursuit, the Emperor leading the way.

The weather was extremely bad, the rain falling in torrents, so that the roads were scarcely passable. The attack of cavalry on the British rear-guard was, therefore, impracticable, but they were much discomfited by the French artillery. About 6 o'clock the air became extremely foggy, so that all further attack was relinquished for the night; but not until the Emperor had ascertained that the whole English army was encamped on the field of Waterloo, in front of the forest of Soignies.

Napoleon, having ascertained the retreat of Blucher on Wavres, and committed the pursuit of him to Marshal Grouchy, believed that the latter was close to the same place,—as he ought to have been; but was not. At 10 o'clock on the night of the 17th the Emperor dispatched an officer to Wavres, to inform Grouchy that there would be a great battle next day; that the English and Belgian armies were posted on the field of Waterloo, its left supported by the village of La Haye; and ordered him to detach seven thousand men, of all arms, and six pieces of cannon, before day break to St. Lambert, to be near to the right of the French army, and co-operate with it; that as soon as Blucher evacuated Wavres, either towards Brussels, or in any other direction, he should instantly march with the rest of his force, and support the detachment sent to St. Lambert. About an hour after this dispatch was sent off, the Emperor received a report from Grouchy, dated from Gembloux at 5 o'clock, stating that "he was still at this village, and had not learned what direction Blucher had taken!"

At 4 o'clock in the morning a second officer was sent to Grouchy to repeat the communication, and the orders which had been sent to Wavres at 10 o'clock. Another dispatch soon after arrived from Grouchy,—who had not at that time been found by either of the officers sent by the Emperor, to state that, "he had learned that Blucher was in Wavres, and would follow him—in the morning!"

The Emperor was now convinced that he had not an hour to spare. He saw the possibility of the Duke's retreat with Blucher through the forest, their subsequent junction, while the great armies of Russia and Austria were about to cross the Rhine and advance on Paris. He now regretted more than ever that he had been unable to attack the English army before the night had intervened, and determined to follow and attack it now, if it commenced a retreat.