William Ponsonby ( British Army officer ) Major - General Sir William Ponsonby KCB ( 13 October 1772 – 18 June 1815 ) , styled The Honourable from 1806 , was an Irish politician and British Army officer who served in the Peninsula War and was killed at the Battle of Waterloo . Background . He was the second son of William Ponsonby , 1st Baron Ponsonby of Imokilly and Hon . Louisa Molesworth . He married Hon . Georgiana FitzRoy , youngest daughter of Charles FitzRoy , 1st Baron Southampton . Between 1796 and 1798 , Ponsonby sat as a Member of Parliament ( MP ) in the Irish House of Commons and represented Bandonbridge . Subsequently he stood for Fethard ( County Tipperary ) and held this seat until the Act of Union in 1801 . He entered the British House of Commons in 1812 , sitting for Londonderry until his death . In 1815 , he was appointed a Knight Commander of the Order of the Bath ( KCB ) . Peninsular War . When Ponsonby ' s command , the 5th Dragoon Guards , arrived in the Peninsula in October 1811 , it became part of John Le Marchant ' s heavy cavalry brigade . For the rest of the war , this brigade included the 5th Dragoon Guards and the 3rd and 4th Dragoons . Ponsonby took part in Le Marchant ' s famous charge at the Battle of Salamanca in July 1812 . On that occasion , the British heavy dragoons rode down one French infantry division and part of a second before being repulsed . Upon Le Marchant ' s death in the battle , Ponsonby took over the brigade , participating in the campaign that included the Siege of Burgos . In 1813 , Ponsonby led his 1 , 200 - strong cavalry brigade at the Battle of Vitoria . During the Battle of the Pyrenees and the fall campaigns in the mountains , the Duke of Wellington sent the bulk of his cavalry to the rear . On 25 January 1814 , Ponsonby took leave of his brigade and in the final battles in France , Lord Charles Manners exercised command . Battle of Waterloo . Ponsonby ' s part in the Battle of Waterloo has been remembered because it highlights some pertinent points about cavalry charging . Ponsonby was in command of the Union Brigade , so - called because it included an English , a Scottish and an Irish regiment . The brigade consisted of the 1st " Royal " and 6th " Inniskilling " Regiments of Dragoons in the first line and the 2nd " Royal North British " Dragoons ( Scots Greys ) in reserve . It had counter - attacked to great effect against the disorganised French columns of Drouet D ' Erlon ' s I Corps . Carried away by their initial success , however , the brigade failed to rally and continued towards the French positions . The Scots Greys in particular , forgetting their supporting role and ignoring the “ recall ” , charged on in disordered groups , some of whom reached the French guns on the other side of the valley . By this time their horses were blown and a swift retribution followed in the shape of a counter - attack by the French Lancers . The brigade suffered very heavy losses ( see table below ) , being effectively destroyed , and it played no further part in the battle . Ponsonby , who was mounted on a horse of less value than the best one in his stable , rode too far and with his horse mired in mud near to enemy lines was set upon by French Lancers . Recognising his rank and worth as a prisoner the French beckoned on him to surrender , but he didn ' t understand them . Then when a group of his own Union Brigade spotted him and rode to his rescue , the Lancers from one of the French line regiments ( either the 3rd Lancers or 4th Lancers ) attached to d ' Erlon ' s I Corps were left with no option but to kill him . Myths growing up after the battle turned this into a guardsman from the " Red Lancers " , which included the surviving squadron of Polish lancers . Other . In the 1970 film " Waterloo , " Ponsonby was played by Michael Wilding . William Ponsonby should not be confused with his second cousin , Frederick Cavendish Ponsonby , a British cavalry officer who also served in the Peninsular War and at the Battle of Waterloo .