What reason?
North Korea, who have already, no, sorry, whose leader has already, stated that he is prepared to start a nuclear war against the U S and South Korea;
Pakistan, who have refused to state that they would never launch a pre-emptive strike against India.
What would happen in the Middle East if Iran reached nuclear weapon capability?
It used to be known as the "nuclear deterrant" and it was the certainty of total global nuclear destruction that kept the USSR and the U S from launching a nuclear war, though it came far too close over Cuba.
I, unfortunately, am not so sure that there is anything that would stop the three countries mentioned above from launching without any thought of the consequences. THEIR perceived enemy would be wiped out and that, unfortunately, would probably justify their action.
The fact that they might just be destroyed by the U S, Russia, the UK or France just might prevent the total destruction of mankind - the Biblical Armageddon!