Lieberman, in explaining his support for the resolution, redefined what it would mean for Iran to achieve nuclear "capability," or the baseline at which we should consider Iran as a nuclear-armed state. "To me, nuclear weapons capability means that they are capable of breaking out and producing a nuclear weapon -- in other words, that they have all the components necessary to do that," Lieberman told reporters. "It's a standard that is higher than saying, 'The red line is when they actually have nuclear weapons.'"

Lieberman's stance on what experts call latent or breakout capability errs on the side of caution, likely in an attempt to give President Obama greater latitude in his dealings with Iran. But for all of Lieberman's good intentions, his argument that the U.S. should use strikes to prevent Iran not just from going nuclear, but from even getting close, is a bad idea. Moving up the U.S.'s red-line in this way would keep Iran one step further from a warhead, sure, but it would also close a potentially useful release valve for U.S.-Iran and Israel-Iran tension.

Broadly speaking, Iran's leadership has three options: give up its nuclear enrichment program now, as Western leaders hope they will; race to the finish in pursuit of an actual nuclear weapon, which would bring the greatest benefits along with the most severe consequences; or compromising for something in between that would minimize the negative consequences while providing some benefits. So far, discussions in the West have concentrated on either extreme, largely ignoring the third choice. But suppose Iran chooses to stop just short of building a bomb. Is that a plausible outcome the United States could accept?

In this hypothetical scenario, Iran would develop the technology and capability for a nuclear weapon without actually building one. They would be close enough to a bomb to feel secure in their deterrent -- if they fear an imminent foreign invasion, as Tehran sometimes does, they could always "break out" and put together a bomb -- but far enough away that the U.S. and Israel wouldn't have to worry about a surprise attack.

Still, it's hard or maybe impossible to say exactly where this middle path -- allowing Iran to move closer to a bomb without actually getting one -- would lead us. How might Israel respond to a latent Iran, for example? Dr. Ali Vaez, director of the Iran project at the Federation of American Scientists, believes breakout capability would meet Israel's conditions for a military strike, but estimates the likelihood of an Israeli attack to be relatively low, at around 20 percent.

"From a cost-benefit point of view," he told me by email, an Israeli strike on a latent Iranian nuclear program "would not achieve much. It will delay the program for a couple of years, but would galvanize Iran to dash toward the ultimate deterrent."