He was essentially a soldier of fortune,[2] and when Portugal applied to Britain for officers to reorganise her army under the Count of Lippe Buckeburg, he was one of the first to volunteer.

[1] Forbes remained in Portugal after the termination of the Seven Years' War (1756–1763); as a Roman Catholic who had married a Portuguese lady, he had no difficulty in getting employment.

[1] When Portugal decided to join the French Revolutionary Wars, Forbes was given command, replacing the Marquis de Minas, his old commander in the 1762 campaigns, who had fallen ill[3] of the 5,000-strong division which, together with a 22-gun brigade of Artillery, was sent as an expeditionary force to assist the Spanish army in the War of the Pyrenees (1793–1795).

Setting sail from the estuary of the Tagus towards the end of September 1793, the expedition, accompanied by several high-ranking volunteers, including Admiral Domingos Xavier de Lima, 7th Marquis of Nice; Hugh Percy, 2nd Duke of Northumberland and the Prince de Montmorency, arrived at Roses on 9 November.

[4] On 27–29 November 1807, Forbes accompanied Queen Mary I, the prince regent, and the court when they fled from the forces led by Jean-Andoche Junot which entered Lisbon.