He has already made it US policy that even if attacked first the US would not respond with nukes if the enemy country did not have their own nuclear weapons. Iran has absolutely nothing at all to worry about Hussain Obama will not even carry out airstrikes against nuclear sites there is certainly no way he would ever dream about nuking Iran especially considering they are a muslim country and he wants the islamic world to love him and dropping a nuke on a muslim country isn't the best way to go about it. If Iran gets nukes they will be made in Iran not America.
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from the article: U.S. Defense Secretary: A nuclear Iran is not inevitable