Jose, you fucking dumbass, who's going to attack the US if they nuke Afghanistan? Nobody, that's who.
For most countries the ability of acquiring nukes is not a technical question, but a political decision. After that political decision has been made, funds are being canalized to the objective of acquiring nukes. Some countries will have the nukes in 1 year like Germany, others after 5 years.
The use of nukes is not justifyable, especially not against Afghanistan.
There comes no existential threat to USA from Afghanistan.
Talibans and Al-Qaeda are no existential threats to the USA.
Some countries will be joining the nuclear club in near future, if USA uses nukes on a country like Afghanistan it will be much more countries joining that nuclear club.
You also seem to be very confident, that USA just nukes a country and no one will do anything against it. Nukeing a country is an extraordinary situation, you can not predict the reactions on such event. It is genocide with no acceptance for it in the world we live in.
Even if there is no military answer to USA, you would be economically boycotted with the conviction manifesting itself that world has to take risks (cutting trade with USA) to accelerate your decline.