While Congress and the American people review the agreement with Iran, there are three fundamental considerations to keep in mind.
The first consideration, as stated by U.S. experts in the 2007 National Intelligence Estimate, is: "Iran has the scientific, technical, and industrial capacity eventually to produce nuclear weapons, if it decides to do so." Nothing in this new agreement - or any other conceivable agreement - can change that fact.
So the Vienna accord is not about returning to an era when Iran lacked the capacity to produce nuclear weapons. Rather, its purpose is to effectively preclude Iran from building a bomb without violating the agreement in ways that we could detect - and that we would have the necessary time to take effective action against Iran. The agreement is being constructed so that Iran never makes the decision to build a bomb or, if it does, is caught early in the process of doing so.
Which leads to the second consideration: An agreement is about buying time.
Today, North Africa, the Middle East, and the Persian Gulf are coming apart at the seams. Libya is unstable and violent. Egypt has swung from revolution to military coup. Saudi Arabia is bombing Iranian-supported Houthi rebels in Yemen. Syria is in a state of civil war, with Islamic State militants gaining ground on the Assad regime. Iraq has lost a third of its territory to the Islamic State.
In assessing the region, Iran is in some cases at least part of the problem - if not the problem. No one thinks otherwise. But none of the region's problems would get better if in the absence of a nuclear agreement, Iran were to sprint to produce enough highly enriched uranium for a bomb - which by some estimates it can now do in two to three months.
A nuclear-armed Iran is likely to believe it has an even freer hand to advance its interests in the Middle East, and it would present a more direct and deadly threat to the United States and its friends and allies.
So an agreement that precludes Iran from building a nuclear bomb buys time to advance American interests without adding a nuclear-armed Iran to the mix. In the long term, the agreement may allow Iran and the United States to develop formulas that reduce tensions in other areas. While this is certainly not a given, it is certainly less likely in the event that Iran deploys nuclear weapons, or if Washington were to try to use military force to forestall Iran from getting a nuclear bomb.
Which leads to the third consideration: The most likely alternative to this agreement is not some other agreement; rather, it is the unraveling of the international coalition in support of sanctions against Iran, less transparency into Iran's activities, and a potential war between the United States and Iran.
If Congress votes to kill this deal, the United States will be widely perceived as scuttling an accord negotiated over many years and supported by Europe, Russia, China, and much of the rest of the world. Support for continuing international sanctions in this environment is unlikely at best.
Moreover, if Congress snuffs out the Vienna agreement, Iran is extremely unlikely to extend the 2013 pact that verifiably freezes much of its nuclear program in place. Rather, Iran is likely to press forward with its nuclear program as it watches support for international sanctions crumble, at some point bumping up against U.S. or Israeli red lines that will ultimately trigger a military strike against Iran. On that day, the United States will be at war with Iran - and largely without allies, who will blame Washington for rejecting a negotiated solution.
War with Iran would severely bend America's international and domestic priorities for a generation, and may only set back, not stop, Iran's nuclear program. A vital region that today is racked with instability will become even more unmanageable. A verifiable agreement that Iran's nuclear program is peaceful would provide a foundation for strategic patience and at least delay if not obviate such a fateful course.
The technical details of this agreement matter. However, what matters at least equally is what is not written down on paper: Iran's capacity to produce a nuclear weapon cannot be erased by any agreement; an agreement with Iran can buy valuable time; and war with Iran is perhaps the most likely alternative to this deal.