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
Just to put things in perspective:

Russia had hard time supplying invasion force that penetrated 100 km deep from its border. And contrary to "Russia sucks" comments, it wasn't just incompetence: supplying invasion forces is HARD. Putin was stupid for thinking he could do Iraq 2003 style invasion without levels of funding that went into Iraq 2003.

Without support of local government, it would be massive effort for Russia to keep a division deployed in USA fed, even before it encounters local resistance.