No, America's influence over its globe-spanning empire is predicated on that. That's not the same thing as it's national security. It would do just fine if it weren't the head of a global hegemony, although it might have to start living within its means.
Security-wise, two oceans, vassal neighbours and two thousand nuclear missiles have it covered.
The U.S. would do just fine but the rest of the world would descend into chaos and we know this for a fact because it happened twice in just the first 50 years of the 20th century.
> Security-wise, two oceans, vassal neighbours and two thousand nuclear missiles have it covered.
You need to take a moment and think about your "vassal states" argument. May I remind you that at the peak of the cold war the US had a hostile country plant their nukes right at the doorstop of the US in one of those neighbors who not so long before was a vassal state?