Though if you're crazy enough to launch a nuke, or carry a nuke in a plane (they're about the size of big car I think), then you're crazy enough to detonate it before you reach you primary target rather than let it do nothing. Especially if you're a terrorist.
Let's not go with the assumption that everyone is completely crazy and only the U.S. is sane. People around the world generally want a lot of the same things. People who pick up arms in one country are often a lot similar to people in another country who do the same. Most people are rational, and most political actors are rational.
Although foot soldiers on any side of any conflict are often indoctrinated to fight and die for a cause (I must emphasize that this applies to just about EVERYONE), the ones pulling the strings are usually pragmatic in their aims, though also often ambitious.
War is not about ideology or fanaticism. War burns up a lot of physical resources, so in order for it to be worth all that effort, you must be able to gain a lot of those resources back. It takes a lot of money to wage a war, and money don't come easy. Zealots are people who stand on street corners with sandwich signs proclaiming the end of the world. They're not the ones with the means to wage a war.
What I'm saying is, don't just assume "everyone is crazy" because it's an easy assumption to make. If we're talking military tactics, it's unwise to think that your enemy is "crazy" and therefore unable to make sound judgments in order to further their own interests.
Is it "crazy" to launch or carry a nuke? Does that make you a crazy terrorist?
What country in the world has ever used nukes on living human targets?
Only one.
Also you can't blow up nukes over other countries. If a country with nukes see's all these American missiles enter their airspace, then suddenly a nuke goes off wiping out a few million of their people and raining down radiation over the rest, then it's bye bye America and all life on Earth.
Okay, well, I see your line of reasoning. But, for those of you who fell asleep during science class, what Teran said boils down to this: Nukes are not gasoline tankards. They don't blow up in response to a spark. That's not how it works.
Nukes have (usually) either uranium or plutonium on the inside. Uranium does not explode when you flick a match at it. What happens inside a nuke is like what happens inside a particle accelerator. A detonation sequence causes a tiny particle to shoot into a piece of uranium, which causes the uranium to split apart (fission). In theory, it would be possible to blow up the whole thing with a conventional missile, destroy the detonator, and prevent that entire fission reaction from happening in the first place. Without fission, the bomb doesn't blow up.
Now, what you say about radiation on the ground when it lands, that, in my understanding, would be a concern. It wouldn't be Hiroshima or Nagasaki, but it'd be a concern.
(This is just a general view based on low-level understanding of science. It isn't my field of specialty, and some of that up there may be outdated.)