If we fail, there's still the hope that other commenters here are right, and Iran isn't intent on using them offensively. If so, then Iran itself will be safe from this sort of attack.

... but it will also be clear to every other that the only way to be secure from Western military intervention is to possess nuclear weapons. There will be a precedent of a country acquiring them despite Western demands and surviving. This will lead to a world where proliferation is rampant, but not necessarily one where their use is no longer taboo as it is today.