Described by the Duke of Wellington, famously critical of his generals, as "the ablest man in the army," Marshal Sir William Carr Beresford was the illegitimate son of the 1st Marquis of Waterford, who rose to be a full general in the British army. Sent to Portugal to rebuild its demoralised forces against Napoleon, Beresford was so successful that Wellington combined the Portuguese and British regiments, with Beresford as commander-in-chief of the Portuguese.

This is the remarkable story of Wellington's strong right arm on whom Wellington relied over many years. Their friendship and trust are revealed in their correspondence, which shows them not only writing to each other almost daily but meeting regularly to discuss strategy or to socialise. It was an amicable and supportive relationship that continued for the rest of their lives, leading to Beresford's appointment as Master General of Ordinance in Wellington's first government in 1828.