During World War I, American Soldiers were referred to as "Doughboys." Why?
The answer is uncertain; it may be related to doughnuts made for soldiers, or to uniform buttons that resembled such doughnuts. What is more certain is that the term dates back to before the Civil War, and was originally used for members of the infantry. During World War I, the usage spread to all American soldiers. For more information, see the The Origins of Doughboy at the Doughboy Center site.
—The Fact Monster