Should We Strike At Iran?

If Iran is developing nuclear weapons, what moral right does the "west" have to prevent it?

If the Canadian government, for whatever reason, decided to build a few nukes for defence, would the United States be justified in flying bombing runs over our facilities and destroying them?

I'm sure many could argue that there is a vast difference between a peaceful democracy like Canada and a one-party rule as in Iran, but I think it is important to ask these questions because I hear a lot about whether the US can stop the Iranian effort and whether or not it is a smart political move, but I rarely hear much discussion of why it is a moral action. What is the "just cause" of preventing countries like Iran from obtaining nuclear weapons?

Just to be clear, I can understand the dire consequences of such a weapon being utilized by a hostile power, especially should these weapons get into the hands of terrorists intend on blowing up civilian populations in Israel or the U.S.. But without hard proof that this would happen, we would be acting on speculation in a pre-emptive strike. Nothing creates enemies faster than persecuting the innocent even if they would be guilty in the future had you not acted.