Napoleon took supreme command of the army, but could not fall back on his old team: Marshal Louis Alexandre Berthier, his former chief of staff, was dead, and he regarded Marshal Joachim Murat as a traitor. Other marshals refused to serve, either because of age or because they had taken an oath of allegiance to the new king, Louis XVIII. Napoleon's appointments in 1815 are often criticized by historians. Marshal Nicolas-Jean de Dieu Soult, an able commander in his own right, became chief of staff, although he had no training for it. Marshal Emmanuel de Grouchy was first to lead the cavalry, for which he was particularly suited. He was later given command of the right wing of the army, although he had never commanded even one corps. The left wing was entrusted to Marshal Michel Ney, whose defection from the Bourbons and transition to Napoleon had been of the greatest importance in the latter's triumphal march to Paris. Ney was considered a fighter, but not a thinker. Marshal Louis-Nicolas Davout, arguably the ablest of the marshals, stayed behind as Minister of War to hold Paris for the Emperor.
Through French sympathizers in the Netherlands, Napoleon had a clear idea of the troop disposition of his enemies. The armies were grouped in loose corps formation. The Prussians were in the Liège - Dinant - Charleroi - Tienen area. Wellington's army, which included Dutch, Hanoverian, Brunswick and Nassau units as well as British, was in the Brussels - Ghent - Leuze - Mons - Nivelles area. The massing of such an army could take days. The supply lines of both armies diverged, those of Wellington ran northwards, those of Blücher eastwards into Germany. In the event of any surprise attack that would have forced them to retreat, the allies would fall back along these routes. Napoleon wanted to strike first one army, then the other, without having to worry about the other. Napoleon's formation was ideally aligned for such a move. Two wings under Ney and Grouchy were to precede the army, and Napoleon was to follow in the centre, throwing the weight on either flank as he chose.
On the 15th of June the French army crossed the frontier into the Netherlands, and at nightfall Napoleon took up his quarters at Charleroi. His army was massed and stood between the allies. During dinner Wellington learned from the Crown Prince of the Netherlands that French scouts had reached Quatre-Bras (19 km south of Waterloo), an important road junction on the army's way to meet Blücher. He had anticipated a bypass on his right flank and had therefore begun to mass his army at Nivelles (Nivelles is 11 km west of Quatre Bras, 22 km west of Sombreffe and 16 km southwest of Waterloo). The Dutch commander at Quatre Bras had realised the importance of the crossroads and disobeyed orders to move to Nivelles. Two brigades now held this crossroads, 33 km from Brussels. It could be held by the Dutch, the Nassau and gradually arriving British and Brunswick reinforcements throughout 16 June (Battle of Quatre-Bras).
Ney, seeing before him a slight slope leading up to the cross-roads, supposed that, though this was held only by weak forces, yet behind it, concealed, lay the allied army in full strength. His experience in Spain had taught Ney to refrain from attacks on Wellington in prepared positions. Thus he made only reconnaissance attacks in the morning, and missed the chance of taking the crossroads before the arrival of the reinforcements. On the same day the Prussians faced the French attack in a previously reconnoitred position and were defeated in the battle of Ligny. Napoleon was unable to win a decisive victory, however, as the French I Corps received conflicting orders on the march from Quatre Bras to Ligny, and thus could not be used at either the Battle of Quatre Bras or Ligny. Thus, the Prussian army was able to avoid annihilation and retreat largely intact.
Field Marshal Blücher had been wounded in the battle and almost captured. The command was therefore led the following night by his Chief of Staff, Lieutenant General von Gneisenau, who ensured that the retreat was not in an easterly direction as expected by the French, but in a northerly direction towards Wavre, from where the Prussians could either come to Wellington's aid or retreat to the east - a decisive factor for the outcome of the later battle.
On the morning of 17 June, Wellington, having learned of the defeat of the Prussians at the battle of Ligny and their retreat to Wavre, set out from Quatre-Bras at 10 o'clock and took up a position between the little town of Braine-l'Alleud and the Meierhof Papelotte. By the morning of June 18 he had drawn up his main force in two divisions on either side of the Charleroi-Brussels road, on a ridge running from west to east. In front of the front of the right wing was the castle of Hougoumont, in the centre the fortified farm of La Haye Sainte, in front of the extreme left wing the farmsteads of Papelotte, La Haye and Fichermont.
Wellington, after the unfortunate outcome of the battle of Ligny, had to expect to be attacked by Napoleon's main force, and therefore confined himself entirely to defence until the arrival of the Prussians. Napoleon had carefully considered the position of his enemy, and had not allowed the troops to set out from their night camps until about 10 o'clock in the forenoon. He placed them about 2 km from the enemy in battle order, so that the infantry formed two meetings, the cavalry a third.
See also: List of French troops of the Armée du Nord