Battle of Waterloo
The grande batterie was too far back to aim accurately, and the only other troops they could see were skirmishers of the regiments of Kempt and Pack, and Perponcher's 2nd Dutch division (the others were employing Wellington's characteristic "reverse slope defence").
The bombardment caused a large number of casualties. Although some projectiles buried themselves in the soft soil, most found their marks on the reverse slope of the ridge. The bombardment forced the cavalry of the Union Brigade (in third line) to move to its left, to reduce their casualty rate.

Napoleon spots the Prussians: At about 13:15, Napoleon saw the first columns of Prussians around the village of Lasne-Chapelle-Saint-Lambert, 4 to 5 miles (6.4 to 8.0 km) away from his right flank—about three hours march for an army. Napoleon's reaction was to have Marshal Soult send a message to Grouchy telling him to come towards the battlefield and attack the arriving Prussians. Grouchy, however, had been executing Napoleon's previous orders to follow the Prussians "with your sword against his back" towards Wavre, and was by then too far away to reach Waterloo.
Grouchy was advised by his subordinate, Gérard, to "march to the sound of the guns", but stuck to his orders and engaged the Prussian III Corps rearguard, under the command of Lieutenant-General Baron von Thielmann, at the Battle of Wavre. Moreover, Soult's letter ordering Grouchy to move quickly to join Napoleon and attack Bülow would not actually reach Grouchy until after 20:00.

First French infantry attack: A little after 13:00, I Corps' attack began in large columns. Bernard Cornwell writes "[column] suggests an elongated formation with its narrow end aimed like a spear at the enemy line, while in truth it was much more like a brick advancing sideways and d'Erlon's assault was made up of four such bricks, each one a division of French infantry". Each division, with one exception, was drawn up in huge masses, consisting of the eight or nine battalions of which they were formed, deployed, and placed in a column one behind the other, with only five paces interval between the battalions.
The one exception was the 1st Division (led by Quiot, the commander of the 1st Brigade). Its two brigades were formed in a similar manner, but side by side instead of behind one another. This was done because, being on the left of the four divisions, it was ordered to send one (Quiot's brigade) against the south and west of La Haye Sainte, while the other (Bourgeois') was to attack the eastern side of the same post.
The divisions were to advance in echelon from the left at a distance of 400 paces apart—the 2nd Division (Donzelot's) on the right of Bourgeois' brigade, the 3rd Division (Marcognet's) next, and the 4th Division (Durutte's) on the right. They were led by Ney to the assault, each column having a front of about a hundred and sixty to two hundred files.

The leftmost division advanced on the walled farmhouse compound La Haye Sainte. The farmhouse was defended by the King's German Legion. While one French battalion engaged the defenders from the front, the following battalions fanned out to either side and, with the support of several squadrons of cuirassiers, succeeded in isolating the farmhouse. The King's German Legion resolutely defended the farmhouse. Each time the French tried to scale the walls, the outnumbered Germans somehow held them off. The Prince of Orange saw that La Haye Sainte had been cut off and tried to reinforce it by sending forward the Hanoverian Lüneburg Battalion in line. Cuirassiers concealed in a fold in the ground caught and destroyed it in minutes and then rode on past La Haye Sainte, almost to the crest of the ridge, where they covered d'Erlon's left flank as his attack developed.
At about 13:30, d'Erlon started to advance his three other divisions, some 14,000 men over a front of about 1,000 metres (1,100 yards), against Wellington's left wing. At the point they aimed for, they faced 6,000 men: the first line consisted of the 1st brigade (Van Bylandt's brigade) of the 2nd Netherlands Division, flanked by the British brigades of Kempt and Pack on either side. The second line consisted of British and Hanoverian troops under Sir Thomas Picton, who were lying down in dead ground behind the ridge. All had suffered badly at Quatre Bras. In addition, Bylandt's brigade had been ordered to deploy its skirmishers in the hollow road and on the forward slope. The rest of the brigade was lying down just behind the road.
At the moment these skirmishers were rejoining their parent battalions, the brigade was ordered to its feet and started to return fire. On the left of the brigade, where the 7th Dutch Militia stood, a "few files were shot down and an opening in the line thus occurred". The battalion had no reserves and was unable to close the gap.

D'Erlon's men ascended the slope and advanced on the sunken road, Chemin d'Ohain, that ran from behind La Haye Sainte and continued east. It was lined on both sides by thick hedges, with Bylandt's brigade just across the road, while the British brigades had been lying down some 100 yards back from the road, Pack's to Bylandt's left and Kempt's to Bylandt's right. Kempt's 1,900 men were engaged by Bourgeois' brigade of 1,900 men of Quiot's division. In the centre, Donzelot's division had pushed back Bylandt's brigade.
On the right of the French advance was Marcognet's division, led by Grenier's brigade, consisting of the 45e Régiment de Ligne and followed by the 25e Régiment de Ligne, somewhat less than 2,000 men, and behind them, Nogue's brigade of the 21e and 45e regiments. Opposing them on the other side of the road was Pack's 9th Brigade, consisting of the 44th Foot and three Scottish regiments: the Royal Scots, the 42nd Black Watch, and the 92nd Gordons, totalling something over 2,000 men. A very even fight between British and French infantry was about to occur.
The French advance drove in the British skirmishers and reached the sunken road. As they did so, Pack's men stood up, formed into a four-deep line formation for fear of the French cavalry, advanced, and opened fire. However, a firefight had been anticipated and the French infantry had accordingly advanced in more linear formation. Now, fully deployed into line, they returned fire and successfully pressed the British troops; although the attack faltered at the centre, the line in front of d'Erlon's right started to crumble. Picton was killed shortly after ordering a counter-attack, and the British and Hanoverian troops also began to give way under the pressure of numbers.
Pack's regiments, all four ranks deep, advanced to attack the French in the road but faltered and began to fire on the French instead of charging. The 42nd Black Watch halted at the hedge and the resulting fire-fight drove back the British 92nd Foot, while the leading French 45e Ligne burst through the hedge cheering. Along the sunken road, the French were forcing the Anglo-allies back, the British line was dispersing, and at two o'clock in the afternoon Napoleon was winning the Battle of Waterloo.
Reports from Baron von Müffling, the Prussian liaison officer attached to Wellington's army, relate that, "After 3 o'clock the Duke's situation became critical, unless the succour of the Prussian army arrived soon".

Charge of the British heavy cavalry: Our officers of cavalry have acquired a trick of galloping at everything. They never consider the situation, never think of manoeuvring before an enemy, and never keep back or provide a reserve.

At this crucial juncture, Uxbridge ordered his two brigades of British heavy cavalry—formed unseen behind the ridge—to charge in support of the hard-pressed infantry. The 1st Brigade, known as the Household Brigade, commanded by Major-General Lord Edward Somerset, consisted of guards regiments: the 1st and 2nd Life Guards, the Royal Horse Guards (the Blues), and the 1st (King's) Dragoon Guards. The 2nd Brigade, also known as the Union Brigade, commanded by Major-General Sir William Ponsonby, was so called as it consisted of an English (the 1st or The Royals), a Scottish (2nd Scots Greys), and an Irish (6th or Inniskilling) regiment of heavy dragoons.

More than 20 years of warfare had eroded the numbers of suitable cavalry mounts available on the European continent; this resulted in the British heavy cavalry entering the 1815 campaign with the finest horses of any contemporary cavalry arm. British cavalry troopers also received excellent mounted swordsmanship training.