Originally Posted by
tartare
They can clearly enrich Uranium. From my limited knowledge - the trickiest bit is not getting a stationary device to explode (a la The Gadget at the Trinity test).
It's miniaturising that device into an operational warhead, which can be successfully mated to a bus attached to the top of a missile. Said warhead and bus needs to be able to withstand the shock of launch, flight and MaxQ/heat of re-entry (up to Mach 20 and 7,000 degrees Celsius), the re-entry vehicle having ablative coatings, and then reliably compress the pit with extreme precision to generate the x-rays and trigger the secondary fusion device.
It seems some have forgotten that the Hiroshima bomb consisted of 64kg of enriched U-235 divided into two lumps, which were driven together in a gun barrel by means of a cordite charge. That's all that's needed to produce a fission (not fusion) explosion. Inefficient, yes; but it killed about 60k people and destroyed a medium sized city. There's no reason to think such a thing wouldn't work as well today as it did in 1945, and if your goal is to create terror and mass casualties, it works perfectly well.
It's not that big and heavy, so there are plenty of ways to deliver it that don't involve missiles - or even aircraft.
I suspect that is why the Powers That Be decided it was too dangerous for the present regime in Iran to possess any quantity of highly enriched U-235...