Others have basically summed it up here - you destroy any useful resources, render the land uninhabitable and spread radiation. You invite everyone else to hit you with anything and everything they have, and you earn the condemnation of many.
Indeed, by using the biggest weapons you have, you have just used the biggest weapons you had.
Understand what this means: Even if you have more, it doesn't matter because you've already said "Hey guys, I'm using these!" You can't scale any threats up from here. Suddenly, anything they do can't make their situation worse, as you're already throwing such a destructive power at them.
It's similar to how, from a government standpoint, ideally the death penalty should, if used at all, be limited to "leading a rebellion against the state", as the laws only exist to help people be part of the system. If you work with the system, you get benefits (for instance, you have money that you put back into the system in return for food you couldn't grow yourself, and you can go to the doctor - yay, society!) If you work against the system, life is harder for you as people don't want to help you, AND you are criminally penalised.
So, with that in mind, if you rob a grocery store, you will be looking at prison time, but might be able to get away with a fine, community service etc. Now if you resist arrest, or take a hostage, they can make it worse for you - suddenly you ARE going to jail. Start shooting at the police, and they might shoot and kill you, but if arrested, your sentence is looking very big. So you always have incentive to cut your losses and not escalate the situation.
On the other hand, if you can be legally executed for minor offences (one ruler in ancient China tried this. The result was rebellion, and the whole government was overthrown and, knowing Chinese history, sliced into tiny pieces, their families put to death in public, and their ancestors dug up and hacked apart), then as soon as they earn the death penalty, it can't get any worse for them and they have no need to stop and give themselves up. They are almost forced to escalate the situation, and there are no consequences for doing so.
This links us back to the nuclear weapons. You are doing your worst, so you can't use any bigger, worse deterrent against them to make them sit down and listen. Suddenly, their best option is to do what, until you started nuking them, was their worst option: launch a full-scale invasion (or use all biological, chemical and nuclear weapons, calling in all allies as well) in the hopes of crushing you to force you to stop.
And even if you win, you've just expended some supply of your best weapon, that you're only too happy to use. Other countries are going to start talking about you.