According to a widely accepted story, in 1813, there was a man from Troy, New York, whose name was Samuel Wilson. During the War of 1812, he worked as a meatpacker, distributing beef to the U.S. Army. When the meat was packaged, Wilson would stamp the barrels with “U.S.” for United States. However, soldiers soon started referring to the beef as “Uncle Sam’s.”
Unlike the robust recruitment efforts of World War I and World War II, recruitment today is more focused on maintaining a standing military. As such there’s more emphasis on character, skill, and career development instead of a call to arms in response to a specific threat.