The USA and Russia certainly have the capability [1], both in terms of technology and in terms of being able to smuggle it in. There were secret caches hidden a

The USA and Russia certainly have the capability [1], both in terms of technology and in terms of being able to smuggle it in. There were secret caches hidden all over Europe by NATO and the USSR, and potentially Soviet caches on US soil, although officially none have been found. But what would they have to gain by having a nuclear weapon in the wild, which is essentially controlled by whoever happens to have possession of it. There are a lot of checks and balances before a normal nuclear weapon can be armed and detonated, most of which you wouldn't be able to have in a backpack nuke. There's no political gain to be had by one of those weapons, if it was detonated it would be fairly obvious who's weapon it was. Conventional weapons have deniability. The problem of reliably getting nuclear weapons onto a target has been largely solved. Between the nuclear triad and MIRV [2] warheads, mutually assured destruction has never been more assured. [1] https://en.wikipedia.org/wiki/Special_Atomic_Demolition_Munition https://en.wikipedia.org/wiki/Special_Atomic_Demolition_Muni... [2] https://en.wikipedia.org/wiki/Multiple_independently_targetable_reentry_vehicle https://en.wikipedia.org/wiki/Multiple_independently_targeta...