I've recently come across the war diary of my great uncle. Despite being an Army Reserve officer, he was given command of a platoon in the 8th Machine Gun Company of the 5th Marines. This seems kind of weird to me, but maybe it happened all the time back then. Thanks.
The 5th Marine Regiment was part of the 4th Marine Brigade during World War I, part of the 2nd Infantry Division. The 2nd Infantry Division was unique in terms of U.S. division structure during World War I in that half of its infantry were U.S. Marines; the Marines had pressed for a ground combat role when the United States entered World War I, and they got their with when two Marine regiments were assigned to the 2nd Infantry Division. The first elements of the division actually went overseas wth the 1st Infantry Division, and the division as a whole was organized in France on October 26, 1917. The 2nd Infantry Division was the first U.S. Army division to be commanded by a U.S. Marine (Brigadier General Charles A. Doyen).
2nd Division Headquarters
3rd Infantry Brigade
9th Infantry Regiment
23rd Infantry Regiment
5th Machine Gun Battalion
4th Marine Brigade
5th Marine Regiment
6th Marine Regiment
6th Machine Gun Battalion
4th Machine Gun Battalion
2nd Trench Mortar Battery
2nd Field Artillery Brigade
12th Field Artillery Regiment
15th Field Artillery Regiment
17th Field Artillery Regiment
1st Field Signal Battalion
2nd Engineer Regiment and Train
2nd Division Train Headquarters and Military Police
2nd Supply Train
2nd Ammunition Train
2nd Sanitary Train (1st, 15th, 16th and 23d Ambulance and Field Hospital Companies)