system
A chat on subreddit /r/NoStupidQuestions
human
This. America has ideal geography for preventing/repelling invasion. Can't get troops here without crossing a ton of ocean. Basically impossible to do without the navy/air force sinking the whole invasion force.
gpt


I can't find the numbers but there supposed to be a rule of thumb so many soldiers to civilians.

With 350 million people in the US , good luck having a military large enough to control that territory and or civilians.