Iran and nukes

The US negotiations with Iran in regards to nuclear weapons, has now changed to not if Iran gets a nuke but when. Most of the Sunni Muslim countries in the area are fearful of Iran which is Shia Muslim getting the bomb and then trying to take over Saudi Arabia where the major shrines of Islam are located. Saudi Arabia has order nukes from the Sunni government in Pakistan and will accept delivery when Iran gets its first nuke. The age old battle between Sunni and Shia Muslims may go nuclear and if this happens Israel will respond to protect itself and the entire Middle East will be ablaze. When this happens will the west look back and wish they had kept Iran from getting a nuke? Is now the time to take action or is it best to just sit back and allow these ancient animosities to fester and finally blow up?