I've seen one or two scenarios set during WWI, but none set during the much more interesting (from a gaming pov) period between 1917 and 1919 when Russia was enveloped by warfare from Finland to the far east. Soldiers, mercenaries and activists from scores of nations came to the aid of one side or the other--White or Red. Battles could spring up anywhere, and the war raged in regions that never saw any part of WWI proper. Chaos, madness and death plus obscure locations and an unusual mix of people from all corners of the globe. All the elements are there.