Next steps to universal nuclear disarmament

It is almost 65 years since the development of the first nuclear bomb, and yet we have had only two cases of use of nuclear weapons in war, namely Hiroshima and Nagasaki. So we have been spared the horror of a large nuclear war during this period when more than 130,000 nuclear weapons were built. This is a very unusual event in the history of mankind: so many weapons built, never to be used. Why has this happened?