Battle of Waterloo
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.
The French were expecting Grouchy to march to their support from Wavre, and when Prussian I Corps (Zieten's) appeared at Waterloo instead of Grouchy, "the shock of disillusionment shattered French morale" and "the sight of Zieten's arrival caused turmoil to rage in Napoleon's army". I Corps proceeded to attack the French troops before Papelotte and by 19:30 the French position was bent into a rough horseshoe shape. The ends of the line were now based on Hougoumont on the left, Plancenoit on the right, and the centre on La Haie.
Durutte had taken the positions of La Haie and Papelotte in a series of attacks, but now retreated behind Smohain without opposing the Prussian 24th Regiment (Laurens') as it retook both. The 24th advanced against the new French position, was repulsed, and returned to the attack supported by Silesian Schützen (riflemen) and the F/1st Landwehr. The French initially fell back before the renewed assault, but now began seriously to contest ground, attempting to regain Smohain and hold on to the ridgeline and the last few houses of Papelotte.
The Prussian 24th Regiment linked up with a Highlander battalion on its far right and along with the 13th Landwehr Regiment and cavalry support threw the French out of these positions. Further attacks by the 13th Landwehr and the 15th Brigade drove the French from Frichermont. Durutte's division, finding itself about to be charged by massed squadrons of Zieten's I Corps cavalry reserve, retreated from the battlefield. The rest of d'Erlon's I Corps also broke and fled in panic, while to the west the French Middle Guard were assaulting Wellington's centre. The Prussian I Corps then advanced towards the Brussels road, the only line of retreat available to the French.

Attack of the Imperial Guard: Meanwhile, with Wellington's centre exposed by the fall of La Haye Sainte and the Plancenoit front temporarily stabilised, Napoleon committed his last reserve, the hitherto-undefeated Imperial Guard infantry. This attack, mounted at around 19:30, was intended to break through Wellington's centre and roll up his line away from the Prussians. Although it is one of the most celebrated passages of arms in military history, it had been unclear which units actually participated. It appears that it was mounted by five battalions of the Middle Guard, and not by the grenadiers or chasseurs of the Old Guard. Three Old Guard battalions did move forward and formed the attack's second line, though they remained in reserve and did not directly assault the Anglo-allied line.

... I saw four regiments of the middle guard, conducted by the Emperor, arriving. With these troops, he wished to renew the attack, and penetrate the centre of the enemy. He ordered me to lead them on; generals, officers and soldiers all displayed the greatest intrepidity; but this body of troops was too weak to resist, for a long time, the forces opposed to it by the enemy, and it was soon necessary to renounce the hope which this attack had, for a few moments, inspired. 

Napoleon himself oversaw the initial deployment of the Middle and Old Guard. The Middle Guard formed in battalion squares, each about 550 men strong, with the 1st/3rd Grenadiers, led by Generals Friant and Poret de Morvan, on the right along the road, to their left and rear was General Harlet leading the square of the 4th Grenadiers, then the 1st/3rd Chasseurs under General Michel, next the 2nd/3rd Chasseurs and finally the large single square of two battalions of 800 soldiers of the 4th Chasseurs led by General Henrion. Two batteries of Imperial Guard Horse Artillery accompanied them with sections of two guns between the squares. Each square was led by a general and Marshal Ney, mounted on his 5th horse of the day, led the advance. Behind them, in reserve, were the three battalions of the Old Guard, right to left 1st/2nd Grenadiers, 2nd/2nd Chasseurs and 1st/2nd Chasseurs. Napoleon left Ney to conduct the assault; however, Ney led the Middle Guard on an oblique towards the Anglo-allied centre right instead of attacking straight up the centre. Napoleon sent Ney's senior ADC Colonel Crabbé to order Ney to adjust, but Crabbé was unable to get there in time.
Other troops rallied to support the advance of the Guard. On the left infantry from Reille's corps that was not engaged with Hougoumont and cavalry advanced. On the right all the now rallied elements of D'Érlon's corps once again ascended the ridge and engaged the Anglo-allied line. French artillery also moved forward in support; Duchand's battery, in particular, inflicting losses on Colin Halkett's brigade. Halkett's front line, consisting of the 30th Foot and 73rd, traded fire with the 1st/3rd and 4th Grenadiers but they were driven back in confusion into the 33rd and 69th regiments, Halket was shot in the face and seriously wounded and the whole brigade having been ordered to pull back, retreated in a mob. Other Anglo-allied troops began to give way as well. A counterattack by the Nassauers and the remains of Kielmansegge's brigade from the Anglo-allied second line, led by the Prince of Orange, was also thrown back and the Prince of Orange was seriously wounded. The survivors of Halkett's brigade were reformed, and engaged the French in a firefight.

I saw the Garde Impériale advancing while the English troops were leaving the plateau en masse and moving in the direction of Waterloo; the battle seemed lost...
The Dutch divisional commander Chassé, on his own initiative, decided at this critical moment to advance with his relatively fresh Dutch division. Chassé first ordered his artillery forward; led by a battery of Dutch horse-artillery commanded by Captain Krahmer de Bichin. The battery opened a destructive fire into the 1st/3rd Grenadiers' flank. This still did not stop the Guard's advance, so Chassé, who was affectionately called "Generaal Bajonet" by his soldiers, ordered his first brigade, commanded by Colonel Hendrik Detmers, to charge the outnumbered French with the bayonet. As the Guard wavered Chassé galloped among his men and found Captain De Haan with a few soldiers of the 19th Militia, whom he ordered into a flank attack. According to Chassé:

[De Haan] jumped over the hedge, reformed the line of about fifty men and the murderous fire he inflicted caused death and confusion among the enemy's lines. He took advantage of their confusion and advanced with the bayonet against them. I had the unspeakable joy to witness 300 Cuirassiers run away from 50 Dutchmen.
The French grenadiers then faltered and broke. The 4th Grenadiers, seeing their comrades retreat and having suffered heavy casualties themselves, now wheeled right about and retired.

To the left of the 4th Grenadiers were the two squares of the 1st/ and 2nd/3rd Chasseurs who angled further to the west and had suffered more from artillery fire than the grenadiers. But as their advance mounted the ridge they found it apparently abandoned and covered with dead. Suddenly 1,500 British Foot Guards under Peregrine Maitland, who had been lying down to protect themselves from the French artillery, rose and devastated them with point-blank volleys. The chasseurs deployed to answer the fire, but some 300 fell from the first volley, including Colonel Mallet and General Michel, and both battalion commanders. A bayonet charge by the Foot Guards then broke the leaderless squares, which fell back onto the following column. The 4th Chasseurs battalion, 800 strong, now came up onto the exposed battalions of British Foot Guards, who lost all cohesion and dashed back up the slope as a disorganized crowd with the chasseurs in pursuit. At the crest the chasseurs came upon the battery that had caused severe casualties on the 1st and 2nd/3rd Chasseurs. They opened fire and swept away the gunners.