I think it's a lot like people elsewhere, have almost no way of conceptualizing what it's like in Seoul, or Tel Aviv, where one is within miles of millions of potential adversaries.
I'm not equating Mexico with NK, or with Syria, but to someone sitting in Pennsylvania, it just doesn't compute.
Perhaps if the poster were to imagine 1,000,000 illegal New Jersey fugitives pouring into Philadelphia every year, that might help them to understand. :)
(with tongue in cheek apologies to those in NJ)
Those who post on FR who are against the war or securing
the border to stop the illegals are no better than those who still deny there was a Holocaust