Uncle Sam n.

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. in abbreviations. "[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. Only gradually superseded earlier Brother Jonathan (1776), largely through the popularization of the figure by cartoonist Thomas Nast. British in World War I sometimes called U.S. soldiers Sammies.