Matt makes all sorts of necessary points here:
Vladimir Putin's warnings against military action against Iran deserve to be taken very seriously...people need to understand that the post-strike diplomatic environment is going to be much more important to the future of the Iranian nuclear program than is any damage that bombing Iran with our on-the-table options might or might not do. If Russia decides to just send some scientists with schematics and materiel over to Iran and show them how to build a nuclear bomb, then -- bam -- nuclear bomb.
Conversely, at the moment not only is Iran under some diplomatic pressure to stop short of weaponizing, many countries around the world are taking direct measures to prevent the Iranians from just easily going and buying the stuff they need. Insofar as an unprovoked American military attack convinces other countries that the real dangerous lunatics live in DC rather than Teheran, countries around the world could cut back on their vigilance and make it much easier for an Iranian nuclear program to succeed.
Right. The more our aggressiveness unsettles the world, the more they'll seek curtail our hegemony, create states able to asymmetrically "balance" our threat, unite against our interests, and throw down markers signaling that we can't take international dominance for granted. Put another way: The more we scare the world, the less they'll cooperate on vanquishing countries we perceive as threatening.
