Go to a good size library, they might have the 20 Vol.
series of History of the First World War by Marshall
Candelvish Publications(they did the W.W.2 series).
Pictures, illustrations, charts, graphs, side articles of W.W.I,
written by the generation who went thru it, and, the ones
afterwards.
Your library might have a loaner series, to take a book home,
or, order it thru the state library system.
I read my series at Buffalo Main Branch at the turn of the
century, took me under two years to go thru the whole series
of books.
Impossible one does not get a feel for The Great War, much
less ideals for House Rules, thanks, Pellulo