What does it make a differece if Iran :
2-does not have nuclear
If the nuclear use in Iran is for domestic electrical-generation purposes, then the only people affected by mistakes are Iranians.
But, if the Iranians intend to develop weapons of mass destruction (WMD) to use on its neighbors, then other nations should prevent materiels and knowledge to be delivered to Iran.
To this second statement, you may ask 'Why?'. The US and USSR had a contest, following World War II (1945) and lasting until the 1980's. This contest almost cost the world its life (i.e. 1962 Cuban missile crisis). The contest bankrupted one economy (USSR) and nearly did the same to the other (US). Only the threat of mutually assured destruction kept the balance of power from tipping into war. Neither nation wanted to see their culture end - which would happen in the aftermath of chaos and disintegration of government.
Nuclear war cannot be won. Everyone on earth loses, because of the radiation contamination which eventually spreads worldwide. This radiation affects generations for centuries thereafter.
People who fail to learn from history will be doomed to repeat it. The cycle of this relearning occurs about every 20-25 years - the length of time it takes for a new generation to grow up and begin filling vital positions in government.