From the Wall Street Journal (warning: long piece):
In his 2002 State of the Union address, President Bush made a promise:
We'll be deliberate, yet time is not on our side. I will not wait on events, while dangers gather. I will not stand by, as peril draws closer and closer. The United States of America will not permit the world's most dangerous regimes to threaten us with the world's most destructive weapons.
In that speech, the president was referring to Iraq, but he has made it clear on a number of subsequent occasions that the same principle applies to Iran. Indeed, he has gone so far as to say that if we permit Iran to build a nuclear arsenal, people 50 years from now will look back and wonder how we of this generation could have allowed such a thing to happen, and they will rightly judge us as harshly as we today judge the British and the French for what they did and what they failed to do at Munich in 1938. I find it hard to understand why George W. Bush would have put himself so squarely in the dock of history on this issue if he were resigned to leaving office with Iran in possession of nuclear weapons, or with the ability to build them. Accordingly, my guess is that he intends, within the next 21 months, to order air strikes against the Iranian nuclear facilities from the three U.S. aircraft carriers already sitting nearby.