Why were American soldiers called Doughboys during the Great War?

American infantrymen in World War I (1914-1918) were nicknamed doughboys, though there is some debate as to the origins of the term. Of course, the U.S. joined the Great War in 1917.

The word was in wide circulation a century earlier in both Britain and America, albeit with different meanings. Horatio Nelson's sailors and the Duke of Wellington's soldiers in Spain, for instance, were both familiar with fried flour dumplings called "doughboys", the precursor of the modern doughnut. Independently, in the United States, the term had come to be applied to bakers' young apprentices, i.e., "dough-boys". In "Moby-Dick" (1851), Herman Melville nicknamed the timorous cabin steward "Doughboy".

https://wiki2.org/en/Doughboy+Newton

A number of theories have been put forward to explain the use of the term "Doughboy" but no one seems sure if any of them is the sole reason. Interested readers are advised to consult the Wikipedia article referenced above.

Though there are many origin stories for "Doughboys," the nickname that finally stuck, there is one with strong historic support. Likely, the name attached early to the Americans from U.S. military operations on the Mexican border.

Reconciliation with Mexico had just concluded in 1916 when marching foot soldiers in Pershing's Expeditionary Force traveled south of the border to fight rebel Pancho Villa. Covered in white adobe dust, the foot soldiers were called "adobes" or "dobies" by mounted troops. Within a few months, these dobies, or Doughboys, were redeployed to Europe.

https://www.theworldwar.org/learn/about-wwi/doughboys