The End? Think "asymmetrical warfare". What are the odds that a nuclear weapon goes off in at least one US city in the next 50 years? I think: greater than 50%. Our era greatly empowers the relatively weak. So although we, euphoric and portly, may be unbeatable in conventional war, we are clearly the target of hostilities from various weak but determined groups. They won't replace us as the fat and happy, but they may give us such a blow that we can't sustain our technologically driven military.
I think ascribing US military power solely to technology leaves out an important factor: the US has continued to flex its military muscle on a regular basis. Yes, a lot of the conficts have been "milk runs", but they still tend to maintain the martial vigor of the troops and the folks at home. This is one reason to look down on the EU, which but for the US would have no reasonable expectation of defending itself in a significant military conflict.
Call me sanguine but I don't think it's so easy to make a nuke and I don't think it can be done without cooperation from a country. I wouldn't advocate that we become less vigilant (in fact, we may have to have more interventions) but I think that keeping tabs on nations that may provide nukes to terrorists is doable. So I would say the chances of a nuclear weapon going off in a US city are very very low.
As for flexing military muscle, you're absolutely right, but since I don't know if that will be sustained, I didn't include it as a factor.
You... are sanguine.
(I... am obliging.)
Once a nuke has been made, it can be sold to a small group without the resources to cook one up from scratch. Some of these groups have a lot of money to throw around, and some of those guarding the nukes are pretty desperately in need of money.
Once acquired, how hard would it be to get a nuclear weapon into the United States. How about bribing the right people in Mexico to allow a shipment to come through unchecked. Sounds fairly plausible, I'd say. Now the nuke is in Mexico. How does it get to the US. Let's take a low-flying plane over the border, maybe straight to a city where the bomb is detonated from the plane, or perhaps landing or parachuting into the desert for a pickup.
Or how about on a container ship, simply detonated while in dock?
The problem with the way such possibilities are typically evaluated, is that they are seen in isolation. The right way to view them is to consider how many nukes are out there (at least tens of thousands), held by how many different kinds of people, sought by how many groups with appropriate financial means, with how many different potential strategies for sneaking them into the US. I think if you want to view the possibility as low you should find one element out of that group and explain why it is an extremely tight bottleneck.
Since it only takes one success for the bad guys to hand us a massive defeat, the odds just stack out heavily against us.