It’s easy to think that America was widely known as the U.S. all the way back to Revolutionary times. Many people didn’t really call themselves Americans, they considered themselves citizens of their states, and their state was associated with others.
In the War of 1812, a contractor named Sam Wilson, a meat packer from New York, was contracted to supply beef and salt pork to the army. He marked the barrels “U.S.”
When someone asked Wilson what that stood for, he laughed and replied “Uncle Sam,” which was his nickname. Within months, the reference went “viral” for the time and showed up in newspapers and cartoons. The first cartoon reference was in 1813.
Pretty soon, all government property was said to be owned by Uncle Sam and troops were known as “Uncle Sam’s boys.”
An artist named James Montgomery Flagg painted the iconic Uncle Sam for World War I posters — modeled on himself.
Uncle Sam was also related to Johnny Appleseed. Wilson married Betsy Mann, who was a cousin of John Chapman, better known as Johnny Appleseed.
Sources:
The Greatest Stories Never Told by Rick Beyer
Various other references