Iran and its six negotiating partners have, at long last, agreed on the key parameters of a final deal regarding Iran's nuclear program. On the basis of the understanding announced in Lausanne, Switzerland, on Thursday, the parties will have three months to fill in the technical details of a Joint Comprehensive Plan of Action.

This breakthrough political understanding between the United States, the other permanent members of the U.N. Security Council plus Germany, and the Islamic Republic of Iran promises to become one of the most consequential and far-reaching nuclear nonproliferation achievements in recent decades.

Once fully negotiated and launched, this deal will block off the options Iran currently has for moving quickly to build nuclear weapons. And the benefits of the deal will extend beyond the particulars of preventing an Iranian bomb. It will also strengthen the worldwide authority of the International Atomic Energy Agency (IAEA) in implementing safeguards on the peaceful development of nuclear energy and give impetus toward the goal of universality in enhanced verification measures such as the IAEA's "Additional Protocol."

In an ideal world, neither Iran nor any other non-nuclear-weapons state would be allowed to possess a complete nuclear-fuel cycle. But Iran is a proud and independent nation, which has had a robust nuclear infrastructure for years. Moreover, its population is solidly behind its nuclear program - a program described by its government as purely peaceful. In the real world, a ban on uranium enrichment cannot be negotiated with Iran; it would have to be imposed.

Iran has suffered serious economic damage from years of sanctions and political damage from international isolation as a result of U.N. resolutions. Yet it gives no indication of being anywhere near capitulation. As New York Times columnist Roger Cohen recently put it, Iran "would rather starve than cave."

There is therefore no point in pursuing what former National Security Council official Gary Sick calls a "unicorn" solution - something that does not exist in the real world. It is time for the critics of diplomacy to acknowledge that the alternative to the kind of deal that Thursday's announcement foreshadows is no deal at all.

Accordingly, the effects of the deal need to be compared with the effects of its absence. A deal would:

Establish verifiable limits on Iran's uranium-enrichment capacity. Without it, Tehran's expansion of enrichment capacity would be unconstrained.

Establish caps on the number and type of uranium-enrichment centrifuges for 10 years. Without it, Iran would soon be able to employ much more efficient centrifuge designs.

Cap uranium enrichment at the 3.67 percent level needed for nuclear-energy reactors. Without it, Iran will be able to enrich to the 90-plus percent level needed for nuclear weapons.

Establish low limits on enriched-uranium stockpiles for 15 years. The lack of a deal implies growth in uranium stockpiles starting this year, including 20 percent-enriched uranium, which Iran was accumulating before the 2013 interim agreement.

End uranium enrichment at Iran's deep underground facility at Fordow for at least 15 years. Without it, Iran could soon increase its enrichment activity there.

Result in modifications to the Arak heavy-water reactor so that it would produce far less weapons-grade plutonium and retain none of it. Without it, Arak would soon be able to produce enough weapons-grade plutonium each year for two bombs.

Allow intrusive, short-notice inspections and enhanced monitoring by the IAEA in perpetuity. Without it, the transparency of Iran's nuclear program would be reduced, increasing the chance that Iran could successfully conduct a clandestine nuclear-weapons program.

At least quadruple the time required for Iran to produce enough highly enriched uranium for a nuclear weapon. Without it, the time it would take Iran to produce enough material for one bomb would shrink to a matter of weeks.

Offer Iran positive incentives to comply with restrictions. Without it, powerful hard-line elements in the regime would continue to make money off of sanction-busting activities.

It has taken many months of intensive negotiations to find a formula for delivering the benefits enumerated above. It will take additional weeks to nail down the complicated details of implementation.

Giving up the ideal solution has been painful for parties on both sides. But it is now time for taking a sober look at what can be gained from a deal and what can be lost without one.

Greg Thielmann is a senior fellow of the Arms Control Association and former intelligence analyst at the Department of State.