While I don't know that we're in any imminent threat of seeing the back half of this come to fruition it's something I've never really seen discussed.
It's this - the United States is in quite a defensible position with respect to its coast lines and would need to lose one border or the other to properly be invaded.
1) As a healthy country with a robust system its geography makes it almost unstoppable.
2) If it ever properly collapsed from within or Gozer the Gozerian set up his ghastly kingdom in NYC we could have a one-continent dark age with little capacity for outside correction.
I've seen people bring up the former but not the later.
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The loneliest part of life: it's not just that no one is on your cloud, few can even see your cloud.