General Alexander Cavalié Mercer (28 March 1783 – 9 November 1868) was a British Army officer of the Royal Horse Artillery.

[1] Mercer's six-gun horse artillery troop arrived too late for the Battle of Quatre Bras, but it fought with the cavalry rearguard covering the army's retreat to Waterloo.

The troop fought on the extreme right wing of Wellington's army at Waterloo, before being moved into the thick of the fighting nearer the centre of the line.

There it beat off repeated charges by French heavy cavalry, disobeying orders to abandon the guns and retire inside nearby infantry squares as the enemy closed.

Mercer's Journal is an important source for historians of the Waterloo campaign, as well as a detailed description of the landscape and people of Belgium and France in the early 19th century.

[6] Mercer was posted to G Troop Royal Horse Artillery around 1806[7] and joined Whitelocke's ill-fated Buenos Aires expedition[4] in 1807.

Arriving on the field of Waterloo, Mercer's Troop briefly took up a firing position on the famous knoll behind the sandpit, which would feature in the fighting the following day.

In mid-afternoon Mercer's Troop was suddenly ordered into the hottest part of Wellingtons' line, between the crossroads and Hougoumont, where its position is now marked by a memorial.

The Grenadiers à Cheval of the Imperial Guard were already emerging through the smoke at the trot as Mercer's guns deployed, so the troop opened fire with case-shot at close range, causing terrible casualties.

To steady his men, Mercer promenaded across his troop's front on horseback, goading the enemy in French and attracting aimed but inaccurate carbine-fire in return.

Shortly afterwards Wellington's infantry advanced, leaving the guns on the ridge to engage masses of French troops in the valley below.

Towards the end of the action a battery established itself on the ridge to Mercer's left and fired into the flank of his troop, causing devastating casualties amongst the limber-horses.

[14] Due to its shortage of horses, the troop was unable to move when the general advance was ordered, and Mercer slept under a limber, amongst the dead and wounded.

[16] Sir Augustus Frazer said, "I could plainly distinguish the position of G Troop from the opposite height by the dark mass of dead French cavalry which, even at that distance, formed a remarkable feature on the field.

Once it had been rejoined by its ammunition and supply wagons, the troop moved off towards Nivelles, leaving some guns and carriages behind for lack of horses.

[3] Recalled to the peacetime army, he served twice in British North America,[21] first as commander of the 6th company of the 5th battalion Royal Artillery at Quebec from 1823.

[22] He served again in British North America from 1837 to 1842,[3] commanding the artillery in Nova Scotia during the 1837 border dispute with the United States[23] which became known as the Aroostook War.

It was written some 30 years earlier, from the original notes Mercer wrote contemporaneously, with additions and verifications from correspondence and other sources.

It covers the period from April 1815 to January 1816, when Mercer returned to Canterbury with D Troop, with an interlude for his leave in England from September to November 1815.