Not much to go on to make an edumakated guess but the devil is in the details.

What does it say on the individual tombstones?  Dates spread out over sixty years?  That would lead me to guess the members of United Spanish War Veterans may have had a fund raiser where they sold plots in two lots reserved for them.  Thats just a guess.

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Let's say the earliest burial was in 1909 and the last was in 1967.  It would make sense.

A second thing to think about is did any of the Illinois Volunteers make it to Cuba, Porto Rico, or the Philippines?  I am certain we would know of stories of a massacre Illinois troopers, ala USS Maine, in the present day if such a thing had occurred.