Notes
This is likely a U.S.M.C. Officer's Service Cap. The braided X-shaped pattern on top of the hat, known as the quatrefoil, has been used to identify officers in the Marine Corps since 1859. During World War I, these hats were intended to be worn behind the front lines for off duty, garrison, or ceremonial duties. They were not typically worn on the battlefield, as overseas caps and steel helmets were found to be far more practical.