This is the World War One (WWI) helmet of Private First Class Robert E. Schalles.
World War One Helmet
"When outside we wear steel helmets for protection against shrapnel. Sometimes large shells burst near our dugout, but one can hear them coming if they are anyways near and have time to duck if a dugout or trench is handy, which is generally the case."
— Robert E. Schalles, April 8, 1918