These countries are in our back yard and sphere of influence and we would definitely stop attackers before they launched (see: cuban missile crisis).
I'm not sure that is a good example.
Let's (for the sake of argument) suppose that relationships between Brazil and Argentina get worse. To the point where is limited military conflict between them. Both of them either develop their own or buy off the market some cruise missiles, actually let's assume they both get lots.
Should the US intervene to stop them? What gives the US the right to interfere?
These are sovereign nations and what right (other than might makes right) does one country have to interfere with others?
But let's suppose the US does interfere - this very intervention would then be the event that kicks off that attack on the US with an overwhelming number of warheads.
I believe the point of the article wasn't to say that there is something to currently worry about,I think it was simply saying that the current situation is potentially flawed.
You CANNOT build something cheaply that will be able to cross the Atlantic or Pacific and still be aimed at a target when it gets here.
These days guidance technology is so cheap it's just not funny. Plenty of amateurs have demonstrated guidance systems on their model planes that cost no more than a few hundred dollars. Let's face it the cost of a guidance system is a GPS receiver, a small processor and a few servo motors.
Now I believe you're right that the complexities of building a plane capable of crossing the Atlantic or Pacific is no mean feat. However I do not believe it is that much more complex than say building a modern car - and there are plenty of countries that are capable of doing that and doing it in huge numbers at low cost.
Again I don't think any of those are currently annoyed at the US enough to do anything like this, but you can bet they have thought of it.