In this undated 5-page letter addressed to "your Royal Highness" Henry Clinton outlines recent changes to Prussian drill and tactical deployment from reports by "Col. Gordon," and then switches to "news from the East." The recipient’s form of address suggests a member of the royal family, and an old note in pencil at the top of Page 1 includes “Gloucester,” suggesting Prince William Frederick, nephew of George III, who would inherit that title, or his father, William Henry, 1st Duke of Gloucester. Given that the letter is filled with military advice, it seems more likely addressed to the younger man, who embarked on a military career in 1789 at the tender age of 13. That approximate date seems confirmed by references to a siege of Belgrade, Turkish forces, and the "Emperor’s grand army," suggesting Emperor Joseph II of Austria and the Austro-Turkish War of 1788-1791, which included the siege and capture of Belgrade from the Turks in 1789. Clinton discusses the new Prussian drill, and advises against accepting it, observing that men are "most likely to follow best and farthest those officers they the most confidence in." He thinks the Turks capable of strong defense behind fortifications, but unable to rally in the open once broken, and expects the Emperor’s army will avoid sieges where an encamped army will be decimated by disease. All appropriate advice for a budding soldier about to step on the world stage in a position of authority. Clinton (1730-1795) had a long military career starting with the Seven Years War and is best known in America for his involvement in the American Revolution from Bunker Hill through Yorktown and serving as British Commander-in-Chief in America from 1777 to 1782. CONDITION: Excellent. Fully legible. With a James Spence Authentication Full letter of authenticity. Ex-Lattimer Collection. Paperwork: JSA LOA