Closing the barn door after the horse has escaped may be futile, but burning down the barn in a fit of pique might be considered an act of lunacy, especially when there is the potential for the horse to come back and bite you in the butt in retribution.
Iran has possessed the technology for separating uranium isotopes for at least six years, and bomb-making can be learned on the Internet. No matter what Israel and the United States do, that bell cannot be unrung. Virusing Iranian centrifuges, murdering Iran's nuclear scientists, and bombing installations will unquestionably slow Iran's nuclear program. But if Iran is hell bent on enriching uranium, even on continuing to the development of a bomb, the outcome is inevitable without regime change. We arranged such a change in Iran in 1954. It left a bad smell then, and can only make a bad U.S. image problem in the region worse than it already is. Thus, such a solution could be a serious mistake if the Iranians themselves aren't leading the effort.
Israel's nervousness is understandable. Iran blusters too much, and if it only means half of what it says, Israel should fear for its safety. However, much mitigates against military action, either by Israel alone, or in consort with the United States.
First, there is Iran's size and population. It has three times as many people as Iraq, and we know what a mess that was. In addition, there is reason to believe that Iranians would coalesce around the government in a time of national peril rather than push for the ouster of the current leaders.
Next, there is the nuclear program itself. It is much more diversified than the Syrian and Iraqi efforts that Israel put out of commission. There is reason to believe that the Iranian government has had North Korean help with tunneling techniques in order to hide installations. The North Koreans are masters of the art of concealment, and need the money.
There is Iran's wealth. The sanctions may be hurting, but there is cheating going on, and we've caught the Iraqis, not very staunch allies after all, as one of the culprits. It appears there are others. This means that Iran is likely to continue having the capital to reduild anything we blow up.
As for nuclear enrichment, Iran is actually as entitled as is any other nation to pursue the technology, even under the Nuclear Non-Proliferation Treaty (NPT). It is Iran's repetitively slimy behavior that has caused much of the rest of the world to oppose and mistrust Iran in the arena of nuclear enrichment. In a strict sense vis-a-vis the NPT, Israel and the United States probably don't have a leg to stand on.
In the wings are China and Russia. While changeable in their pronouncements,they are not going to help for numerous reasons. This means that the United Nations will be impotent in this issue.
Finally, trouble in the Persian Gulf could lead to disruption of a third of the world's oil supply. The price of crude would head toward $150-200 a barrel. That is no way to run an economic recovery.
The simplest, and probably most potent, approach would be a policy pronouncement by the United States extending its nuclear umbrella. It should be made unequivocal that we would retaliate decisively (yes, that means even the Big One, if necessary) should Iran or its surrogates use Iranian nuclear material in an attack against another party. No excuse by Iran about stolen or misplaced material would be acceptable, since they were warned not to go there in the first place. This is the "If you play, you pay" rule. We would rely on our increasing skill in fingerprinting nuclear material to discover its origin. Iran would know that it faced becoming a porcelain hole in the ground the day after an event. Its neighbors could take refuge in the American pledge, thus foregoing the need to field their own nukes.
The key to this policy working is the continuing relevance of deterrence. It worked through the Cold War, and it continues to work today. And don't kid yourself; the Iranians aren't crazy. They do not consider Israel's destruction worth their own disappearance.
Diplomacy should continue to be the favored path. But the right stick might make the carrot taste a lot better.