If you want to understand the roots of our hobby you have to understand wargames. In the 70s just before D&D exploded in popularity and became a fad, the hobby was dominated by wargamers crossing over.
This is a excellent history written by one of the principles behind SPI an old wargame company.
I was probably part of the last generation of young gamers to start out with wargames exclusively. Note that I live (and still live) in a small town in a rural area of NW Pennsylvania. We were big enough to support speciality stores. We were enough out of the way that we lagged a couple of years behind as far as general avalibility of games. I remember everybody my age or older starting out on wargames. (circa 1978) However younger gamers invariably started out on D&D. I put the tipping point about 80-81.