Here are the facts, as I see them:
1. The international sanctions that are currently in place were directed solely towards Iran’s nuclear program, not at its support of Shiite military forces elsewhere in the Middle East.
2. The approval of these sanctions was a largely unrecognized diplomatic victory for the Obama Administration which created the leverage for the nuclear deal.
3. Our relationships with Russia and China are, at best, complicated, so we cannot rely on their good will in this process. Russia in particular has gone out of the way in the past to play spoiler in the Middle East. It supports the Assad regime and, by implication, Iranian efforts to prop it up.
4. Therefore, there is absolutely no reason to believe that Russia and China would agree to reinstate, and even strengthen, sanctions against Iran in the event the US repudiates the agreement to which they have given their consent.
5. Without the support of the entire international community, it is preposterous to think that sanctions can fundamentally alter Iranian behavior.
6. As a result, the only way to eliminate the Iranian nuclear program is through war, or at least a thoroughly credible threat of war. I know of no reason why it can be safely assumed that the Iranian regime would cave in the face of an ultimatum, and if one were issued and rejected, we would have no choice but to go to war, probably without much international support. The questions, then, are what kind of war would be fought, and what would our objectives be?
7. It is assumed in some quarters that a few surgical air strikes would be sufficient to eliminate the Iranian nuclear infrastructure. In fact, the process would be much slower and messier than that; it would initially require the destruction of the Iranian air defense system, and any attempt to target the regime’s ability to build centrifuges would involve bombing civilian areas. (Thanks for an article by Zack Beauchamp on Vox for this insight)
8. The Iranians would undoubtedly retaliate. Under other circumstances, it could be assumed that this would include a missile attack on Israel by Hezbollah, but Hezbollah may be too overstretched at the present time to help. Terrorist attacks on American military personnel in Iraq would be likely. The big question is whether Iran would threaten to close the Straits of Hormuz.
9. Closing the Straits would be a low-tech operation, using small boats and mines. It would be very difficult to prevent, particularly in the early stages of the conflict. A complete and immediate blockade of the Iranian coastline would be required to avoid it. Even the mere threat of closure would cause the price of oil to skyrocket, and would do untold damage to the US economy.
10. Even if the air strikes appeared to be successful, we would not have sufficient intelligence to know exactly how much damage was done. As a result, we would probably have to follow up on a regular basis. In other words, we would be committing to “cutting the grass” for an indefinite period of time.
11. Assuming, for purposes of argument, that the Iranian nuclear program could be wiped off the face of the earth, that would not eliminate Iranian support of Shiite military groups throughout the Middle East. Fundamental revisions to the objectives of the Iranian state cannot be accomplished without regime change, which appears to be the goal of the Israeli government and many US hawks.
12. Bombing will not result in regime change. It will increase public support in Iran for the regime, and (probably) even for the nuclear program.
13. There are two plausible ways to bring about regime change. The first is an Iraq-style invasion and long-term occupation, with all of the attendant costs, which would be far greater than they were in Iraq, given the larger size of the country to be occupied. The second, if you want to avoid that, would be the complete destruction of the country, probably through the use of nuclear weapons.
The nuclear deal is the product of negotiations, not an ultimatum, so it is not and could not be perfect. The real question is whether there is a better alternative. If our objective is limited to the destruction of the nuclear program, the alternative is air strikes for an indefinite period of time. If our objective is regime change, it means an invasion and occupation or a nuclear attack. I do not believe these are better options.