Causing fusion is so damned difficult. Look how long the North Korean scientists worked hard at it and couldn't figure it out. Now imagine a bunch of goat herders living in caves trying to do the same thing. Unless they somehow lay their hands on an old Cold War suitcase nuke (which has such low yield it would barely be bigger than a conventional weapon), I don't see it happening.
There is that.....however, knowing a bit about nuclear weapons, I can say that the Soviets were never big on
maintaining theirs......and the most common (and smallest) design that was used was the "synchronous shock-wave compression" type, which unless receiving constant maintenance, will simply not work.
The idea that some clown selling a compact cold war warhead out of the back of a pickup truck.....and having it actually function is on its face ludicrous.
That said, there ARE designs that are simple enough to build in a well-equipped machine shop......all that is required is a critical mass of weapons-grade U-235 or Plutonium, however, building one without killing everything within a mile (except the cockroaches), isn't easy either.
Further, smuggling one into a port with sufficient shielding to evade a radiation sensor (and virtually all ports use them) has some practicality issues as well.
It's far more likely that some homegrown jihadis can get their hands on enough radioactive medical material, or reactor waste products to build a "dirty bomb", than it is to consider the probability of an actual weapon detonation.
It isn't impossible however, particularly with the rapidly eroding stability of Pakistan.
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