Battle of Waterloo
Uxbridge then ordered a charge by three squadrons of the 3rd Hussars of the King's German Legion. They broke through the French cavalry, but became hemmed in, were cut off and suffered severe losses. Meanwhile, Bachelu's and Tissot's men and their cavalry supports were being hard hit by fire from artillery and from Adam's infantry brigade, and they eventually fell back.
Although the French cavalry caused few direct casualties to Wellington's centre, artillery fire onto his infantry squares caused many. Wellington's cavalry, except for Sir John Vandeleur's and Sir Hussey Vivian's brigades on the far left, had all been committed to the fight, and had taken significant losses. The situation appeared so desperate that the Cumberland Hussars, the only Hanoverian cavalry regiment present, fled the field spreading alarm all the way to Brussels.

French capture of La Haye Sainte: At approximately the same time as Ney's combined-arms assault on the centre-right of Wellington's line, rallied elements of D'Erlon's I Corps, spearheaded by the 13th Légère, renewed the attack on La Haye Sainte and this time were successful, partly because the King's German Legion's ammunition ran out. However, the Germans had held the centre of the battlefield for almost the entire day, and this had stalled the French advance.
With La Haye Sainte captured, Ney then moved skirmishers and horse artillery up towards Wellington's centre. French artillery began to pulverise the infantry squares at short range with canister. The 30th and 73rd Regiments suffered such heavy losses that they had to combine to form a viable square.

The possession of La Haye Sainte by the French was a very dangerous incident. It uncovered the very centre of the Anglo-allied army, and established the enemy within 60 yards of that centre. The French lost no time in taking advantage of this, by pushing forward infantry supported by guns, which enabled them to maintain a most destructive fire upon Alten's left and Kempt's right ...
The success Napoleon needed to continue his offensive had occurred. Ney was on the verge of breaking the Anglo-allied centre.
Along with this artillery fire a multitude of French tirailleurs occupied the dominant positions behind La Haye Sainte and poured an effective fire into the squares. The situation for the Anglo-allies was now so dire that the 33rd Regiment's colours and all of Halkett's brigade's colours were sent to the rear for safety, described by historian Alessandro Barbero as, "... a measure that was without precedent".
Wellington, noticing the slackening of fire from La Haye Sainte, with his staff rode closer to it. French skirmishers appeared around the building and fired on the British command as it struggled to get away through the hedgerow along the road. The Prince of Orange then ordered a single battalion of the KGL, the Fifth, to recapture the farm despite the obvious presence of enemy cavalry. Their Colonel, Christian Friedrich Wilhelm von Ompteda obeyed and led the battalion down the slope, chasing off some French skirmishers until French cuirassiers fell on his open flank, killed him, destroyed his battalion and took its colour.
A Dutch–Belgian cavalry regiment ordered to charge retreated from the field instead, fired on by their own infantry. Merlen's Light Cavalry Brigade charged the French artillery taking position near La Haye Sainte but were shot to pieces and the brigade fell apart. The Netherlands Cavalry Division, Wellington's last cavalry reserve behind the centre having lost half their strength was now useless and the French cavalry, despite its losses, were masters of the field, compelling the Anglo-allied infantry to remain in square. More and more French artillery was brought forward.

A French battery advanced to within 300 yards of the 1/1st Nassau square causing heavy casualties. When the Nassauers attempted to attack the battery they were ridden down by a squadron of cuirassiers. Yet another battery deployed on the flank of Mercer's battery and shot up its horses and limbers and pushed Mercer back. Mercer later recalled, The rapidity and precision of this fire was quite appalling. Every shot almost took effect, and I certainly expected we should all be annihilated. ... The saddle-bags, in many instances were torn from horses' backs ... One shell I saw explode under the two finest wheel-horses in the troop down they dropped
French tirailleurs occupied the dominant positions, especially one on a knoll overlooking the square of the 27th. Unable to break square to drive off the French infantry because of the presence of French cavalry and artillery, the 27th had to remain in that formation and endure the fire of the tirailleurs. That fire nearly annihilated the 27th Foot, the Inniskillings, who lost two thirds of their strength within that three or four hours.

The banks on the road side, the garden wall, the knoll and sandpit swarmed with skirmishers, who seemed determined to keep down our fire in front; those behind the artificial bank seemed more intent upon destroying the 27th, who at this time, it may literally be said, were lying dead in square; their loss after La Haye Sainte had fallen was awful, without the satisfaction of having scarcely fired a shot, and many of our troops in rear of the ridge were similarly situated.
During this time many of Wellington's generals and aides were killed or wounded including FitzRoy Somerset, Canning, de Lancey, Alten and Cooke. The situation was now critical and Wellington, trapped in an infantry square and ignorant of events beyond it, was desperate for the arrival of help from the Prussians. He later wrote, The time they occupied in approaching seemed interminable. Both they and my watch seemed to have stuck fast.

Arrival of the Prussian IV Corps: Plancenoit: Night or the Prussians must come.

The Prussian IV Corps (Bülow's) was the first to arrive in strength. Bülow's objective was Plancenoit, which the Prussians intended to use as a springboard into the rear of the French positions. Blücher intended to secure his right upon the Châteaux Frichermont using the Bois de Paris road. Blücher and Wellington had been exchanging communications since 10:00 and had agreed to this advance on Frichermont if Wellington's centre was under attack. General Bülow noted that the way to Plancenoit lay open and that the time was 16:30.
At about this time, the Prussian 15th Brigade (Losthin's) was sent to link up with the Nassauers of Wellington's left flank in the Frichermont-La Haie area, with the brigade's horse artillery battery and additional brigade artillery deployed to its left in support. Napoleon sent Lobau's corps to stop the rest of Bülow's IV Corps proceeding to Plancenoit. The 15th Brigade threw Lobau's troops out of Frichermont with a determined bayonet charge, then proceeded up the Frichermont heights, battering French Chasseurs with 12-pounder artillery fire, and pushed on to Plancenoit. This sent Lobau's corps into retreat to the Plancenoit area, driving Lobau past the rear of the Armee Du Nord's right flank and directly threatening its only line of retreat. Hiller's 16th Brigade also pushed forward with six battalions against Plancenoit.
Napoleon had dispatched all eight battalions of the Young Guard to reinforce Lobau, who was now seriously pressed. The Young Guard counter-attacked and, after very hard fighting, secured Plancenoit, but were themselves counter-attacked and driven out. Napoleon sent two battalions of the Middle/Old Guard into Plancenoit and after ferocious bayonet fighting—they did not deign to fire their muskets—this force recaptured the village.

Zieten's flank march: Throughout the late afternoon, the Prussian I Corps (Zieten's) had been arriving in greater strength in the area just north of La Haie. General Müffling, the Prussian liaison to Wellington, rode to meet Zieten.
Zieten had by this time brought up the Prussian 1st Brigade (Steinmetz's), but had become concerned at the sight of stragglers and casualties from the Nassau units on Wellington's left and from the Prussian 15th Brigade (Laurens'). These troops appeared to be withdrawing and Zieten, fearing that his own troops would be caught up in a general retreat, was starting to move away from Wellington's flank and towards the Prussian main body near Plancenoit. Zieten had also received a direct order from Blücher to support Bülow, which Zieten obeyed, starting to march to Bülow's aid.
Müffling saw this movement away and persuaded Zieten to support Wellington's left flank. Müffling warned Zieten that "The battle is lost if the corps does not keep on the move and immediately support the English army." Zieten resumed his march to support Wellington directly, and the arrival of his troops allowed Wellington to reinforce his crumbling centre by moving cavalry from his left.