After seeing action in the Second Anglo-Burmese War at Pegu (now known as Bago, Myanmar) in December 1852 and again in January 1853, he became an assistant commissioner in Tenasserim (now known as the Tanintharyi Region) and was severely wounded in 1856-57 while fighting insurgent Karens and Shans in the Yunzalin District.

In 1866, he went to Mandalay as agent of the chief commissioner, and in August of that year had a narrow escape from a body of insurgents who had murdered three of the royal princes.

During the disturbances that ensued he embarked nearly all the Europeans and other Christians at the Burmese capital on board a river steamer and brought them safely to Rangoon, for which he received the thanks of the governor-general.

In May 1867, he exerted his influence with the king to prevent the execution of three young princes, two of whom owed their lives to his intercession, the other having been beheaded before a reprieve arrived.

In a speech on 17 February 1886, the governor-general, Lord Dufferin, made special mention of ‘Colonel Sladen, to whose courage and knowledge of the people we are so much indebted for the surrender of the king’.

[3] Through his daughter Marion, he was a grandfather of Lionel Fielden (1896–1974), who was educated at Eton and Brasenose College, Oxford before becoming a successful producer at the BBC.

[8] He owned Villa Massei, a 16th-century hunting lodge and 60 acres estate in Massa Macinaia, near the ancient walled city of Lucca, Italy.