He has been described as one of the finest British cavalry commanders of his generation; he was also an intellectual soldier who had a great influence on the efficient functioning of the army he served in.

His father, John Le Marchant, was a Cornet in the Royal Dragoons who attended Pembroke College, Oxford, and served with distinction under the Marquess of Granby during the last three campaigns of the Seven Years' War.

[2] After leaving Dr Morgan's school in Bath characterised as "one of the two greatest dunces that had ever been there" (the other being Sir Sidney Smith), Le Marchant reformed his character and was commissioned in a regiment of the Wiltshire Militia.

[7] Le Marchant served as a brigade major during the disastrous Low Countries campaign of 1793–95, and for a time had command of his regiment as the most senior officer present.

[11] The sword exercise became quite celebrated, and the elderly king, George III, became familiar with it, and country lanes abounded with small boys practising the cuts with sticks.

[12] Le Marchant toured Britain teaching cadres, drawn from both regular and yeomanry cavalry units, his system of swordsmanship; his methods were practical and painstaking and he was himself a superb mounted swordsman.

Le Marchant was also to have gone to Ireland to teach his sword exercise there but was prevented from doing so, so his brother-in-law, Lt. Peter Carey (16th Light Dragoons), undertook this duty in his stead.

Though a good relationship existed between himself and Paget, Le Marchant found it difficult to keep company with the immensely wealthy and fashionable peer.

Le Marchant was the first lieutenant-governor of the college, and during the nine years that he held this appointment he trained many officers who served with distinction under Wellington in the Peninsular War.

It is notable that a number of senior serving officers, such as General Robert Ballard Long, attended courses at the college in order to improve their military knowledge.

During the development of the Anglo-Portuguese attack on the over-extended French left wing Wellington is reported as saying to Le Marchant that he must take the first favourable opportunity to engage the enemy's infantry, "You must then charge at all hazards" was his final instruction.

Le Marchant, knowing he had achieved a magnificent success, was leading a squadron against the last of the formed French infantry when he was shot and his spine broken.

[18] Wellington's despatch after the battle stated: "the cavalry under Lieutenant-General Sir Stapleton Cotton made a most gallant and successful charge against a body of the enemy's infantry, which they overthrew and cut to pieces.

[20] He wrote several treatises on cavalry tactics and other military subjects,[15] most published semi-anonymously (the ones adopted as army regulations were not officially ascribed to a single author).