Forbes was a soldier of fortune who enlisted in the Portuguese army in the Seven Years' War and afterwards settled in Portugal, remaining in the service for the rest of his life. He was the second son of George Forbes of Skellater, Aberdeenshire and although he never owned the property (which his father sold) he was frequently referred to, and known as Forbes de Skellater, or Forbes-Skellater. The manuscripts refer to the campaigns of the Portuguese army in the wars against France of 1793-1795 and 1801 (for which he was recalled from retirement).

The manuscripts appears to have once been part of a larger military archive which had been put in order between 1858 and 1862, but which had subsequently been broken up and the files relating to Forbes separated from the rest.