In the Battle of Campo Maior, or Campo Mayor (an older spelling most often used in English language accounts), on 25 March 1811, Brigadier General Robert Ballard Long with a force of Anglo-Portuguese cavalry, the advance-guard of the army commanded by William Beresford, clashed with a French force commanded by General of Division Marie Victor de Fay, marquis de Latour-Maubourg.

[2] On 15 March, Marshal Édouard Mortier and 4,500 troops belonging to the V Corps laid siege to Campo Maior Castle.

The only units to see action were the 13th Light Dragoons, the 1st and 7th Portuguese Cavalry Regiments, and part of Cleeves' KGL artillery battery, a total of 700 sabres and two cannon.

The historian Sir John Fortescue wrote, "Of the performance of Thirteenth, who did not exceed two hundred men, in defeating twice or thrice their numbers single-handed, it is difficult to speak too highly.

[6][7] Beresford's decision to call off his troops when they appeared to be in a position to destroy or force the surrender of the entire French column was taken by his detractors as an early sign of the lack of military insight he was to show later in the campaign at the Battle of Albuera.

[8] Following Beresford's halting of his troops, the French infantry continued unmolested along the road and, having been passed by the returning allied light cavalry, easily recaptured the convoy and successfully escorted it into Badajoz.

Beresford also claimed that his taking personal command of the heavy dragoon brigade had prevented Long from ordering them to attempt a suicidal charge against French infantry squares.

[11] Long was of the opinion, and was subsequently supported in this by the historian Sir William Napier,[12] that if Beresford had released the British brigade of heavy dragoons he would have been able to drive off the remaining French cavalry (two squadrons who had not been charged by the 13th LD and some rallied fugitives), who were in close support of their infantry, and consequently force the French infantry to surrender.

[14] Wellington, after receiving Beresford's report on the clash at Campo Maior, issued a particularly harsh reprimand to the 13th LD calling them "a rabble" and threatening to remove their horses from them and send the regiment to do duty at Lisbon.