The increasing likelihood that Iran will successfully acquire a nuclear weapon cannot be avoided. While this author does not predict with certainty that it will happen, nor believe the US and EU shouldn’t do everything in its power to stop it, it is a real possibility. Therefore, policy discussions should also begin to think about the potential implications of a nuclear Iran and consider how the US and others can mitigate the effects thereof.
North Korea has not yet set off a cascade of proliferation, largely because the potential new nuclear states are all US allies (South Korea, Japan, perhaps Taiwan). The same is not true of all the Middle East. In addition, states may react not only to Iran, but to their perceived reaction of others, i.e. Egypt reacting to Iran directly as well as to its belief Saudi Arabia will respond with its own program. Below is a regional map of what a nuclear Iran could lead to:

And to give readers something else to think about, the following countries are currently giving serious consideration to nuclear energy in the near term (within 10 years): Azerbaijan, Belarus, Egypt, Indonesia, Kazakhstan, Norway, Poland, Estonia, Latvia, Lithuania, Turkey, Vietnam.
As if that were not troubling enough, the following countries have long term plans or studies underway: Algeria, Australia, Chile, Georgia, Ghana, Jordan, Libya, Malaysia, Morocco, Namibia, Nigeria, Bahrain, Kuwait, Oman, Saudi Arabia, Qatar, UAE, Syria, Venezuela and Yemen.
This is not to say that all or even most will move forward. Nuclear energy (even for peaceful purposes) requires a massive financial investments in infrastructure, technology, training and more. Additionally, some states may decide against it at various states of their programs, as has happened many times in the past.
Therefore, in the event that Iran does indeed acquire a nuclear weapons capability, which we’ll define as the generally agreed perception that Iran does (whether or not it has tested), the United States and Europe need to think long and hard about instruments of state power which can be applied to prevent a cascade of nuclear weapons states and/or to ensure that peaceful nuclear energy (a legal right of every NPT signatory) stays peaceful and secure.