How effective have efforts to control the spread of arms (including biological and chemical), especially nuclear weapons, have been in the past?
In some ways, it is hard to say how effective these efforts have been.
On the one hand, we can argue that they have been very effective. The proof of this is in the fact that so few countries have actually acquired weapons of mass destruction. For example, outside of the five nuclear powers at the time of the creation of the Nuclear Nonproliferation Treaty, only four other countries (India, Pakistan, North Korea, Israel) either have or are believed to have weapons of mass destruction. This tends to imply that the nonproliferation effort has worked.
However, we can also argue that there is no connection with the nonproliferation regime. We can say that the countries that have not gotten WMD have done so simply because they do not want to have them. These countries have not been deterred from getting WMD. Instead, they have simply voluntarily chosen not to get them.
So, we can clearly say that not many countries have gotten WMD. However, we cannot say with so much certainty that it was the nonproliferation efforts that caused them to forego getting WMD. They may have done so simply because they do not feel the need to have them.