Why were American soilders known as doughboys?

Answer 1

Before the US entered WWI, in 1916, the Army sent a "punitive expedition" into northern Mexico chasing Francisco "Pancho" Villa and his crowd of bandits. Mexico was in chaos from a decades long civil war. Villa was a bloodthirsty, ruthless individual, who one murderous night had led his crew over the border into Columbus, New Mexico and slaughtered the inhabitants. But he was also a candidate for President of Mexico. So the idea was to chase Villa, but not necessarily catch him, as that might have been embarrassing. The punitive expedition was under the command of General John J. Pershing, who the next year would become head of the American Army sent to France.

On the punitive expedition the marching infantry were constantly coated with white adobe dust stirred up by their marching feet in the dry climate. The mounted soldiers called these dusty foot soldiers "adobes". From there it was a short step to "dobies", and from there, by metathesis to "Doughboys".

By the way, the name remained in use even in WWII. You'll hear people claim that WWII soldiers referred to themselves as "GI"s. But their letters show that more often they referred to themselves as "Doughs", short for Doughboys, which their fathers had been.