United States Army Divisions of World War I - good overview from Wikipedia
General
World War One Draft Registration Cards, 1917-1918
Registers of Enlistments in the U. S. Army, 1798 - 1914
Applications for Headstones for Military Veterans, 1925-1941
General Index to Pension Files, 1861-1934
Remarried Widows Index to Pension Applications, 1887-1942 (covers service 1812-)
Veterans Administration Pension Payment Cards, 1907-1933 these must be browsed - primarily Civil War onward
Washington: Washington State Dept. of WWI Service Statement Cards, 1917-1919