Sometimes this question sneaks up to me like an intrusive thought, but recent events have made me give this question a lot more consideration.
"Comply with us, or we will kidnap your leader, bomb and occupy your land. Resist further and we will use nuclear weapons."
They have no need to dress their words in the clothes of decency and legality. They hold the power and technology to force anyone to comply, even if a country had nuclear weapons too such factor would only become relevant if the country had a nuclear triad and a stockpile over 2 digits.
Operation Northwoods and September 11th prove that the leadership of the U.S. would gladly welcome violence on Americans if it can be used as justification for further violence against enemies of the state, combine that with the knowledge that the leadership of the U.S. is the only one in the world that has proven itself capable of using nuclear weapons on civilians. They are bandits unshackled by morality, humanity or consequences.
There would be no international consequences, there is no collective of states in the world strong enough and willing to challenge the U.S. if it started making demands backed by the threat of nuclear bombardment.
There would be no domestic consequences, the U.S. has the most obedient population in the world as well as the world's most advanced police state.
So, what is stopping the U.S. from holding the world hostage with a nuclear gun?
Venezuela, Cuba, Mexico, Brazil. It could start annexing all of the Western Hemisphere tomorrow as enslaved resource colonies and the world couldn't stop them. It could force every country in the world except Russia and China to pay half their economy as tribute to keep American capitalism going for another 200 years and all it would need to do so is the threat of carpet bombing and nuclear terrorism.
The rate of profit will continue to fall either way, capitalism is inherently unsustainable, it will cease to exist no matter if it takes 50 years or 300. But in the short term I see it as a looming possibility that the administration of the U.S. will drop all pretenses and just take what they want whenever they want, backing their threats with nuclear weapons as it desperately seeks more resources to plunder, like a dying vampire thirsting for blood.
While it's true that they haven't used a nuclear weapon since then, you have to keep into consideration that modern day nuclear weapons are much more powerful, travel further and are much more difficult to stop, making the use of such weapons even more unpredictable and USA is not the only nation with them.
If there is one thing USA is good at it's war games. That's the only planning they do. And they absolutely know it's much better to drop hundreds of thousands of small weapons instead of 1 big one, if your goal is civilian targets there's much less risky weapons that accomplish the same thing. The purpose of nuclear weapons is the maintenance of sovereignty. If your state doesn't have them you are not independent.