That should be the case but it's not that easy though. Some countries do have a missle defense system but it's never been tested in combat. The average long range ICBM can reach any point on the earth in about a half hour I think. It's like hitting a bullet with another bullet. It's still a bit of a question mark whether it will actually work every time (only one can make a significant number of casualties).
NATO countries in Europe should be protected by the new missile defense system but I've read a lot experts state that it could fail miserably.

You'd still want to try to disable those defense systems before launching an ICBM, and most countries don't have the capability of doing that.


Another way is to use bomber planes, protected by fighters, to drop them. That's no option for non-western powers, they're no match.

Then you have nuclear submarines, which is tricky because you don't know where those fuckers are exactly. They fire short range missiles which are hard to intercept, especially if you don't know where they come from. But only six (and possibly Israel) countries have those.

With countries like Iran and North Korea it's a purely political question. In contradiction to what a lot of hysterics say: North Korea and Iran have proven to be 'rational' actors when it comes to international politics. It's utter suicide for those regimes to try to use their (future) nuclear arsenal and they know this as well.
The US, and others don't want Iran to become a nuclear power because of the power leverage Iran would get. Because you obviously don't want to fuck around with nuclear powers, it's still risky. (Cuban crisis)