Battle of Waterloo
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. They were, however, inferior to the French in manoeuvring in large formations, were cavalier in attitude, and, unlike the infantry, some units had scant experience of warfare.
The Scots Greys, for example, had not been in action since 1795. According to Wellington, though they were superior individual horsemen, they were inflexible and lacked tactical ability. "I considered one squadron a match for two French, I didn't like to see four British opposed to four French: and as the numbers increased and order, of course, became more necessary I was the more unwilling to risk our men without having a superiority in numbers."
The two brigades had a combined field strength of about 2,000 (2,651 official strength); they charged with the 47-year-old Uxbridge leading them and a very inadequate number of squadrons held in reserve. There is evidence that Uxbridge gave an order, the morning of the battle, to all cavalry brigade commanders to commit their commands on their own initiative, as direct orders from himself might not always be forthcoming, and to "support movements to their front". It appears that Uxbridge expected the brigades of Sir John Ormsby Vandeleur, Hussey Vivian, and the Dutch cavalry to provide support to the British heavies. Uxbridge later regretted leading the charge in person, saying "I committed a great mistake", when he should have been organising an adequate reserve to move forward in support.

The Household Brigade crossed the crest of the Anglo-allied position and charged downhill. The cuirassiers guarding d'Erlon's left flank were still dispersed, and so were swept over the deeply sunken main road and then routed.

The blows of the sabres on the cuirasses sounded like braziers at work.
Sir Walter Scott, in Paul's Letters to his Kinsfolk, described the following scene:Sir John Elley, who led the charge of the heavy brigade, was [...] at one time surrounded by several of the cuirassiers; but, being a tall and uncommonly powerful man, completely master of his sword and horse, he cut his way out, leaving several of his assailants on the ground, marked with wounds, indicating the unusual strength of the arm which inflicted them. Indeed, had not the ghastly evidence remained on the field, many of the blows dealt upon this occasion would have seemed borrowed from the annals of knight-errantry [...]Continuing their attack, the squadrons on the left of the Household Brigade then destroyed Aulard's brigade. Despite attempts to recall them, they continued past La Haye Sainte and found themselves at the bottom of the hill on blown horses facing Schmitz's brigade formed in squares.
To their left, the Union Brigade suddenly swept through the infantry lines, giving rise to the legend that some of the 92nd Gordon Highland Regiment clung onto their stirrups and accompanied them into the charge. From the centre leftwards, the Royal Dragoons destroyed Bourgeois' brigade, capturing the eagle of the 105e Ligne. The Inniskillings routed the other brigade of Quoit's division, and the Scots Greys came upon the lead French regiment, 45e Ligne, as it was still reforming after having crossed the sunken road and broken through the hedge row in pursuit of the British infantry. The Greys captured the eagle of the 45e Ligne and overwhelmed Grenier's brigade. These would be the only two French eagles captured by the British during the battle. On Wellington's extreme left, Durutte's division had time to form squares and fend off groups of Greys.

As with the Household Cavalry, the officers of the Royals and Inniskillings found it very difficult to rein back their troops, who lost all cohesion. Having taken casualties, and still trying to reorder themselves, the Scots Greys and the rest of the Union Brigade found themselves before the main French lines. Their horses were blown, and they were still in disorder without any idea of what their next collective objective was. Some attacked nearby gun batteries of the Grande Battery. Although the Greys had neither the time nor means to disable the cannon or carry them off, they put very many out of action as the gun crews were killed or fled the battlefield. Sergeant Major Dickinson of the Greys stated that his regiment was rallied before going on to attack the French artillery: Hamilton, the regimental commander, rather than holding them back cried out to his men "Charge, charge the guns!"
Napoleon promptly responded by ordering a counter-attack by the cuirassier brigades of Farine and Travers and Jaquinot's two Chevau-léger (lancer) regiments in the I Corps light cavalry division. Disorganized and milling about the bottom of the valley between Hougoumont and La Belle Alliance, the Scots Greys and the rest of the British heavy cavalry were taken by surprise by the countercharge of Milhaud's cuirassiers, joined by lancers from Baron Jaquinot's 1st Cavalry Division.

As Ponsonby tried to rally his men against the French cuirassers, he was attacked by Jaquinot's lancers and captured. A nearby party of Scots Greys saw the capture and attempted to rescue their brigade commander. The French lancer who had captured Ponsonby killed him and then used his lance to kill three of the Scots Greys who had attempted the rescue.
By the time Ponsonby died, the momentum had entirely returned in favour of the French. Milhaud's and Jaquinot's cavalrymen drove the Union Brigade from the valley. The result was very heavy losses for the British cavalry. A countercharge, by British light dragoons under Major-General Vandeleur and Dutch–Belgian light dragoons and hussars under Major-General Ghigny on the left wing, and Dutch–Belgian carabiniers under Major-General Trip in the centre, repelled the French cavalry.
All figures quoted for the losses of the cavalry brigades as a result of this charge are estimates, as casualties were only noted down after the day of the battle and were for the battle as a whole. Some historians, Barbero for example, believe the official rolls tend to overestimate the number of cavalrymen present in their squadrons on the field of battle and that the proportionate losses were, as a result, considerably higher than the numbers on paper might suggest.
The Union Brigade lost heavily in both officers and men killed (including its commander, William Ponsonby, and Colonel Hamilton of the Scots Greys) and wounded. The 2nd Life Guards and the King's Dragoon Guards of the Household Brigade also lost heavily (with Colonel Fuller, commander of the King's DG, killed). However, the 1st Life Guards, on the extreme right of the charge, and the Blues, who formed a reserve, had kept their cohesion and consequently suffered significantly fewer casualties. On the rolls the official, or paper strength, for both Brigades is given as 2,651 while Barbero and others estimate the actual strength at around 2,000 and the official recorded losses for the two heavy cavalry brigades during the battle was 1,205 troopers and 1,303 horses.

Some historians, such as Chandler, Weller, Uffindell, and Corum, assert that the British heavy cavalry were destroyed as a viable force following their first, epic charge. Barbero states that the Scots Greys were practically wiped out and that the other two regiments of the Union Brigade suffered comparable losses. Other historians, such as Clark-Kennedy and Wood, citing British eyewitness accounts, describe the continuing role of the heavy cavalry after their charge. The heavy brigades, far from being ineffective, continued to provide valuable services. They countercharged French cavalry numerous times (both brigades), halted a combined cavalry and infantry attack (Household Brigade only), were used to bolster the morale of those units in their vicinity at times of crisis, and filled gaps in the Anglo-allied line caused by high casualties in infantry formations (both brigades).
This service was rendered at a very high cost, as close combat with French cavalry, carbine fire, infantry musketry, and—more deadly than all of these—artillery fire steadily eroded the number of effectives in the two brigades. At 6 o'clock in the afternoon the whole Union Brigade could field only three squadrons, though these countercharged French cavalry, losing half their number in the process. At the end of the fighting, the two brigades, by this time combined, could muster one squadron.
Fourteen thousand French troops of d'Erlon's I Corps had been committed to this attack.