N.A.T.O. Policy has always been, and by that I mean France and the United Kingdom, that if ANYONE uses a nuclear, chemical or biological weapon against us WE WILL go NUCLEAR. If Saddam Hussain had used such weapons we would have removed Baghdad from the map. And that policy has been set in stone for at least 40 years. That is because we equate chemical and biological with nuclear.

Note: British policy on the use of nuclear weapons is that the United Kingdom forswears first use of such weapons unless:
1) Such weapons are used against us. Under some circumstances that would include their threatened use.
2) If the safety or survival of the state is threatened.

Arguably any country will the ability to manufacture nuclear weapons has a duty to itself to do so. And ensure that it has the means to deliver them. The same arguement applies to chemical and biological weapons.

The big issue is not the weapons themselves but the intentions of the countries concerned. The most obvious case is that of Iran. Iran considers Israel to be a hostile power. One that it considers to have no right to exist. It is stated Iranian policy that it will destroy Israel if it is able to do so. However Israel is a nuclear power. Israel has always refused to confirm or deny it's nuclear status. Given that the Arab countrys have not made common cause and obliterated Israel, it is overwelming likely that Israel has nuclear weapons and is more than prepared to use them.
However: At present their is no check on Israel deciding to using them as a first strike. Unless other nuclear powers make it clear that they will be attacked. It therefore follows that Iran has to develop weapons as a check.

Now here's the question. Will Iran develop them but foreswear first use or develop them and use them in a genocidal attack?