The Origin of Uncle Sam FROM The Online Etymology Dictionary

---symbol of the United States of America, 1813, coined during the war with
Britain as a contrast to John Bull, and no doubt suggested by the initials U.S.
"[L]ater statements connecting it with different government officials of the
name of Samuel appear to be unfounded" [OED]. The common figure of Uncle Sam
began to appear in political cartoons c.1850.