Origin and history of U.S.

also U.S.A., abbreviation of United States of America, in use by 1814 in addresses, etc.; not common otherwise before c. 1920. Before then it often meant United States Army. Extended colloquial form US of A is by 1973.

symbol of the government of the United States of America, 1813, coined during the second 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. In late 19c. He gradually superseded Brother Jonathan, largely through the work of cartoonist Thomas Nast (1840-1902). In World War I Sammy sometimes was a British word for a U.S. soldier, but it apparently had no currency among the Americans.