The chief benefit of the agreement was to block Iran’s pathways to the bomb by freezing or reducing its capacity to produce the amounts of fissile materials required to do so—the most difficult step in the bomb-making process. Thus, as we know from the additional IAEA inspectors and 24/7 on-site cameras deployed as part of the agreement, Iran today operates only some 5,000 older-model centrifuges, maintains a much-reduced stockpile of enriched uranium, limits its centrifuge research and development programs, and has filled the core of its heavy-water nuclear reactor with concrete. Whereas experts assessed that, at the time of the deal, Iran was only months from being able to produce enough nuclear material for a bomb, under these new terms it is now at least a year away—plenty of time for the international community to anticipate any oncoming danger and respond accordingly.

Where would Iran be today without the agreement? It’s hard to know for sure, but even if Tehran had continued only to steadily expand its nuclear program as it had for the previous two decades, it would today likely be operating the more than 20,000 centrifuges it had at the time of the agreement. Iran would have continued enriching uranium and building its stockpiles, and it would’ve been operating a fully functional heavy-water nuclear reactor capable of producing enough plutonium for one or two nuclear weapons per year, all without the additional verification provisions put in place to ensure this was all it was doing.

What that means: Without the deal, Iran would today likely be only weeks from possessing enough weapons-usable material for a bomb. And without the verification procedures Iran committed to in the agreement, the international community would have no reliable way of knowing if it was stockpiling that material—until it was too late.

Critics assert that tougher sanctions and the threat of military force could have prevented Iran from arriving at this point. This argument does not hold up to scrutiny. Economic sanctions, even when progressively tightened over the years, never halted Iran’s program. When George W. Bush took office in 2001, Iran possessed only a few hundred centrifuges and no stockpile of enriched uranium. In the years that followed, Bush expanded U.S. sanctions and obtained three UN Security Council resolutions that all imposed real pressure on Iran. Yet when he left office in 2009, Iran had nearly 6,000 centrifuges, over 2,000 pounds of low enriched uranium (LEU), a partly built heavy-water reactor, and a secret underground enrichment facility.

Even after international sanctions were dramatically expanded and toughened in 2010 through 2012 under Obama, Iran continued expanding the quantity of its centrifuges, grew its enriched-uranium stock to more than 30,000 pounds, and build its heavy-water reactor—right up until the November 2013 Joint Plan of Action froze that program, paving the way for negotiations. It is wishful thinking to imagine that Iran, after expanding its program for decades despite heavy sanctions, would have miraculously decided in 2016 or 2017 to suddenly unilaterally refrain from any further nuclear advances, or even abandon its program altogether.