Deterrant?, I honestly do not fear Russia, NK or any other nation but the U.S in starting a nuclear war, why? for the simple fact that some idiot think tank somewhere has convinced themselves that such a war in winnable.
We haven't won a war against a peer enemy since WW2 and lost most of the wars against non-peers since then.
Sure, but effectively because the US is fighting on terms that nuclear weapons do not really help: The US has more than enough conventional firepower to turn most of the countries it has been at war in to sheets of glass, but the goal is very distinctly to not do that.
Losing wars fought in the streets where you're trying to not kill civilians isn't really a scenario where adding nukes helps.
(google and android and apple and iphones might all be examples, as might be science like CRISPR-CAS9, none of which originate in Chinese or Russian labs, though China was smart enough to build factories as a service, for examples, but in some collaboration of european cultures including in the americas).