system
A chat on subreddit /r/NoStupidQuestions
human
You must also remember, there are 2-3 million veterans in this country that have served in combat post 9/11. Large groups of "normal people" can become extremely dangerous to a foreign invader, especially when blending in to familiar terrain that we like to hunt and fish year round.
gpt
But also remember that most veterans barely fire personal weapons, and a very very small fraction have seen actual combat.   That said, I believe that military basic is hugely helpful... Especially American military training.  It changes you, in ways that allow you to become that fighter, if need be.  It inoculates against fear.  It drives home the basic understanding of initiative, etc. Etc. 

My opinion is that many millions of people who never served would become fierce and talented fighters in their own right, given foreign occupation.  Sort of like the "undiscovered talent" of combat.  I'd go back to Louisiana if I had a magic bus and time to build a squad.  Can't tell you how many street smart guys there are (except instead of street, I mean in the wild), and who can shoot squirrels in the head at 50 meters with a .22.  

Also, critically, they know win NOT to shoot, so you can bag something bigger.  ;)