DENVER – The agreement reached in Geneva between Iran and the five permanent members of the United Nations Security Council plus Germany (the so-called P5+1) is an excellent start to the difficult process of dissuading Iran from attempting to become the world’s newest nuclear-weapons power. It is too early to praise the deal as an historic achievement, but it is also far too soon to peg it as a failure, or to suggest that better negotiators somehow could have done a better job of wrangling concessions from their Iranian counterparts.