And COVID19 is turning that assumption on its head. The only question that remains is whether the US can buttress what's left of its public health institutions to keep it at bay, and prepare for the next wave that comes, whether of this pandemic or the next one.
Borders are absolutely a defense, they're just not an absolute defense.
That SARS-CoV-2 was able to spread globally as a pandemic has more to do with its transmissibility and latency characteristics.
Which is what proves that the US isn't unassailable from a pandemic. It would be a mistake to assume that SARS-CoV-2 or something with similar ability to breach geographical barriers won't occur again. Also, nature doesn't care about political entities, and a pandemic could also start in the US.
Budgeting is a zero sum game.
Stockpiles of essential PPE, sure. Pervasive monitoring for early detection? Not worth it.
That's 'magical thinking'.
No doubt the Japanese thought pretty much the same in 1942. That's why the Dolittle Bombing Raid on Tokyo was such a shock to their systems.
Even in 1945, with the Imperial Japanese Navy suppressed or destroyed and their supply chain in tatters, an invasion would have been incredibly costly.
Occupying enemy territory, across an ocean, with a large population of civilians disinclined to cooperate, is never easy.
This was discussed by Max Brooks on Fresh Air yesterday:
https://www.npr.org/2020/03/24/820601571/all-of-this-panic-c...
What Brooks doesn't discuss is the extent to which those countries have varying degrees of authoritarianism or the tendency for citizens to voluntarily comply with public orders.
In the US however, our military stockpiling is geared toward propping up defense industries. This has value to a point, but only when its objectives are actually defense preparedness, but not when the objectives are profit maximization.
The military is no substitute for having properly resourced and coordinated national institutions to deal with huge public health issues.