Paul Pillar offers a dose of sanity on negotiations with Iran:

It would be a grave mistake for the United States and its partners to adhere to an inflexible negotiating position based on the mistaken notion that an Iran bent on building nuclear weapons can be coerced or physically prevented from doing so. If Iran really were bent on building the bomb, it probably would have done so by now, based on a nuclear program that dates to the time of the shah.

One of the more common claims that Iran hawks make is that Iran is “hellbent” on acquiring nuclear weapons, which means that Iran will keep seeking this no matter how much it costs. That’s an odd way to describe what Iran has been doing. If Iran is so intent on getting nuclear weapons, it has certainly taken its time, and by all informed accounts it has not decided that it wants to have them. The policies that Iran hawks favor as a means of “prevention” would not be able to discourage Iran from acquiring nuclear weapons if it were truly “hellbent” on this goal, and an armed attack would give Iran added incentive to seek nuclear weapons. If Iran is not “hellbent” on doing this, and the evidence suggests that it is not, there should not be too many obstacles to reaching a nuclear deal that satisfies the U.S. and Iran.

Pillar continues:

Certain prevention is impossible, whether through diplomacy, military force or other means. Like many other things in international relations, this issue is a matter of weighing relative risks. There is minimal risk of Iran throwing away an agreement that ended debilitating sanctions to pursue a nuclear weapons scenario that so far exists more in U.S. fears and politics than in Iranian desires.

We should also recognize that it isn’t all that accurate to frame the choice on Iran policy in terms of “prevention” or containment. So-called prevention won’t definitely prevent anything, and will in all likelihood bring about the outcome it is supposed to “prevent,” while containment of a nuclear-armed Iran may prove unnecessary.